Summer Sp Target Information

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SUMMER SP TARGET INFORMATION

ALGIEBA (g LEO)

BASIC INFORMATION OBJECT TYPE: Binary Star CONSTELLATION: Leo BEST VIEW: Late April DISCOVERY: Known to Ancients DISTANCE: 131 ly BINARY SEPARATION: 4” (170 AU) ORBITAL PERIOD: ~500 yr. APPARENT MAGNITUDE: 1.98

NOTABLE FEATURES/FACTS • William Herschel discovered Algieba’s binary nature in 1782. • Both components of Algieba have evolved beyond the main sequence. They began their lives as B-type stars, and they will end their lives as white dwarfs. • In 2010, a team including former UT astronomer Arte Hatzes discovered a planet orbiting Algieba A. The planet is nine times the mass of Jupiter and orbits the star in 1.2 years at an average distance of 1.2 AU. SUMMER SP TARGET INFORMATION

MESSIER 97 (THE OWL NEBULA)

BASIC INFORMATION OBJECT TYPE: Planetary Nebula CONSTELLATION: Ursa Major BEST VIEW: Early May DISCOVERY: Pierre Mechain, 1781 DISTANCE: ~2000 ly DIAMETER: 1.8 ly APPARENT MAGNITUDE: +9.9 APPARENT DIMENSIONS: 3.3’

DISTANCE DETERMINATION The distances to most planetary nebulae are very poorly known. A variety of methods can be used, providing mixed results.

In many cases, astronomers resort to statistical methods to estimate the distances to planetary nebulae. Although we don’t have accurate distances for most of the planetary nebulae in the Milky Way, we do know exactly how far away the Large Magellanic Cloud is. There are lots of planetary nebulae in the LMC, and we know their distances because we know the distance to the LMC. So, astronomers measure physical properties (such as mass, temperature) of lots of planetary nebulae in the LMC. Using this information, astronomers can create a mathematical relationship between certain observed properties and the distance. This method is based on assumptions that are still up for debate, and thus is subject to errors of as much as a factor of two or three.

NOTABLE FEATURES/FACTS • M97 is known as the “Owl Nebula” because an early drawing by Lord Rosse in 1848 resembles the face of an owl.

Figure 1: Drawing by Lord Rosse, 1848 • Analysis of the expansion rate of the nebula suggests the progenitor star died out 6000 to 8000 years ago. • The central star is a white dwarf estimated to have slightly more than half the mass of the Sun. It shines at an apparent magnitude of +15.8, but is at least 50 times as luminous as the Sun. • Messier added this object to his catalog in 1781, shortly after its discovery by his friend Pierre Mechain. SUMMER SP TARGET INFORMATION

THE LEO TRIPLET (M65, M66, NGC 3628)

BASIC INFORMATION OBJECT TYPE: Galaxy Group CONSTELLATION: Leo BEST VIEW: Early May DISCOVERY: Charles Messier, 1780 (M65 & M66) William Herschel, 1784 (NGC 3628) DISTANCE: 41.1 million ly (M65) 33.6 million ly (M66) 37.8 million ly (NGC 3628) DIAMETER: ~117,000 ly (M65), ~89,000 ly (M66), ~163,000 ly (NGC 3628) APPARENT MAGNITUDE: 10.3 (M65), 9.7 (M66), 9.5 (NGC 3628)

DISTANCE DETERMINATION Cepheid Variables: Cepheids are a type of standard candle. The variation in the brightness of the star over time directly relates to its luminosity. Comparing this to the star’s apparent magnitude yields the distance. This method was used to determine the distance to M66.

Tully-Fisher Relation: The rotational characteristics of spiral galaxies are directly related to their luminosities. So, by measuring the rotation of a galaxy and using this relationship, astronomers can calculate the galaxy’s intrinsic brightness. This method was used to determine the distances to M65 and NGC 3628.

NOTABLE FEATURES/FACTS • All three galaxies in the triplet are spirals. M65 and M66 are seen at oblique angles (M65 is the upper one in the image). NGC 3628 is seen almost edge on. • These three galaxies appear to have interacted hundreds of millions of years ago. NGC 3628 and M66 display the most significant signs of past interaction. • M65 contains a generally older stellar population than its neighbors but may be undergoing a new round of star formation. • M66 displays an unusual spiral arm pattern that may be a result of its encounter with NGC 3628. • NGC 3628 displays a dense dust band that is likely the result of its interaction with M66. • In 2014, astronomers detected a star-forming clump in the trio that may be a Tidal Dwarf Galaxy. This type of galaxy forms from the tidal debris of a galactic interaction. • In 2015, astronomers discovered a dense star cluster in NGC 3628 that may be similar to our own Omega Centauri. The cluster appears to have formed from the debris of a disrupted dwarf galaxy, lending credence to the hypothesis that some of the Milky Way’s globular clusters were formed or captured during galactic interactions. • Charles Messier added M65 and M66 to his catalog on the night of 1 March 1780. SUMMER SP TARGET INFORMATION

NGC 4565

BASIC INFORMATION OBJECT TYPE: Galaxy CONSTELLATION: Coma Berenices BEST VIEW: May DISCOVERY: William Herschel, 1785 DISTANCE: 41 million ly DIAMETER: ~190,000 ly APPARENT MAGNITUDE: +10.4 APPARENT DIMENSIONS: 16’ x 3’

DISTANCE DETERMINATION One way to determine the distance to a galaxy is to figure out its intrinsic brightness. Once astronomers know this, they can observe how bright the galaxy appears, and then use the inverse square law of light to calculate the distance. The Tully-Fisher Relation quantifies the connection between the rotational characteristics of spiral galaxies and their luminosities. By measuring the rotation of a galaxy and using this relationship, astronomers can calculate the galaxy’s intrinsic brightness, and thus, its distance.

NOTABLE FEATURES/FACTS • NGC 4565 is also known as “The Needle Galaxy.” • This galaxy is an edge-on spiral. Infrared observations confirm it also has a central bar structure. • NGC 4565 is similar in size and structure to the Milky Way. This galaxy is a nice example of what the Milky Way would look like if seen edge-on from about 40 million miles away. • NGC 4565 hosts a supermassive black hole at its center. The black hole is estimated to be about three million times the mass of the Sun. • This galaxy is moving away from the Milky Way at a speed of 2.8 million miles per hour. SUMMER SP TARGET INFORMATION

MESSIER 104 (THE SOMBRERO GALAXY)

BASIC INFORMATION OBJECT TYPE: Galaxy CONSTELLATION: Virgo BEST VIEW: Late May DISCOVERY: Pierre Mechain, 1781 DISTANCE: ~31 million ly DIAMETER: ~80,000 ly (based on 31 Mly distance) APPARENT MAGNITUDE: +8.0 APPARENT DIMENSIONS: 9’ x 4’

DISTANCE DETERMINATION The most commonly used method to obtain a distance to M104 is by measuring fluctuations in the surface brightness of the galaxy. Galaxies are made up of individual stars. The stars cannot be resolved at very great distances, but sensitive instruments can detect graininess in the galaxy’s texture. The more distant the galaxy, the less grainy its texture appears. Astronomers are able to quantify these surface brightness fluctuations and their relationship to distance. The above value is an average of recent measurements using the surface brightness fluctuation method.

NOTABLE FEATURES/FACTS • M104 is classified as a spiral galaxy, but it also has some characteristics of an elliptical galaxy. For example, it is “bulge dominated,” meaning it has an unusually large galactic bulge relative to the size of its disk. • We are viewing M104 just six degrees from perfectly edge-on. • A prominent dust ring encircles the galaxy. This ring and the dominant galactic bulge give the galaxy the appearance of a large hat. • The dust ring is the primary location of new star formation in M104. • The Sombrero Galaxy contains at least 1900 globular clusters… the most of any known galaxy. • M104 has a supermassive black hole at its center. Astronomers at the University of Texas have measured its mass at 660 million times the mass of the Sun. • The Sombrero Galaxy is moving away from us at a speed of about 1000 km/s (about 2.2 million mph). • Charles Messier never officially added this object to his catalog. A hand- written note in his personal catalog mentions that he observed the galaxy on 5/11/1781; the same day Mechain discovered it. The galaxy was added to the catalog as entry number 104 in 1921. SUMMER SP TARGET INFORMATION

MESSIER 64 (THE BLACK EYE GALAXY)

BASIC INFORMATION OBJECT TYPE: Galaxy CONSTELLATION: Coma Berenices BEST VIEW: Late May DISCOVERY: Edward Pigott, 1779 DISTANCE: ~17 million ly DIAMETER: ~50,000 ly APPARENT MAGNITUDE: +8.5 APPARENT DIMENSIONS: Approx. 10’ x 5’

DISTANCE DETERMINATION Two methods are commonly used to determine the distance to M64. Both are actually methods to determine the intrinsic brightness of the galaxy (or stars within it). Comparing this absolute magnitude to the observed brightness of the galaxy (or stars) yields the distance.

Tip of the Red Giant Branch: Stars at the “tip of the red giant branch” have evolved to a point where they are just beginning to fuse helium in their cores. These stars have a known intrinsic brightness.

The distance value adopted above is based on the mean value of distances obtained using these two methods.

NOTABLE FEATURES/FACTS • M64 gets its nickname due to the appearance of a prominent dark dust band. It is also sometimes called the Sleeping Beauty or the Evil Eye galaxy. • The galaxy contains two counter-rotating disks of gaseous material. The inner disk extends about 3000 ly from the galaxy’s core. The outer disk extends from there to the outskirts of the galaxy. • A new wave of star formation is underway as a result of friction between the counter-rotating gas disks. • Messier added this object to his catalog on the night of 1 March 1780. SUMMER SP TARGET INFORMATION

MIZAR & ALCOR

BASIC INFORMATION OBJECT TYPE: Multiple Star System CONSTELLATION: Ursa Major BEST VIEW: Early June DISCOVERY: Known to Ancients DISTANCE: 83 ly BINARY SEPARATION: ALCOR & MIZAR: 11.8’ (0.3 ly) MIZAR A & B: 14.4” (367 AU), ALCOR A & B: 1.1” (28 AU) ORBITAL PERIOD: ALCOR & MIZAR: 750,000 yr MIZAR A & B: 5000 yr +, ALCOR A & B: 90 yr + APPARENT MAGNITUDE: MIZAR: 2.2, ALCOR: 4.0

NOTABLE FEATURES/FACTS • Mizar and Alcor were known as the “horse and rider” in Arabic astronomy. The pair is thought to symbolize marriage in Indian astronomy. • Historically, Mizar and Alcor are said to have been used as tests of vision. • Mizar and Alcor form a naked-eye double, but it wasn’t until 2009 that astronomers determined the pair are likely gravitationally bound. • In 1617, Benedetto Castelli observed Mizar in his telescope and believed he detected a companion. He wrote to Galileo, asking him to confirm the discovery. Mizar thus became the first telescopic binary to be discovered. • Mizar A was the first spectroscopic binary discovered (Edward Charles Pickering, 1889). The stars that make up Mizar A orbit one another in about 20.5 days. • Mizar B is also a spectroscopic binary. The stars that make it up orbit one another in about six months. • In 2009, astronomers looking for planets orbiting Alcor found an M-dwarf star orbiting the star instead. This brought the total number of known stars in the system to six. • All of the stars in the group except for Alcor’s companion are main-sequence A-type stars. They are on average about 20 times brighter than the Sun and have surface temperatures ranging from 13,000° F to 16,000° F.

SUMMER SP TARGET INFORMATION

NGC 5139 (w CENTAURI)

BASIC INFORMATION OBJECT TYPE: Globular Cluster CONSTELLATION: Centaurus BEST VIEW: Early June DISCOVERY: Ptolemy, 150 A.D. DISTANCE: 15,800 ly DIAMETER: 175 ly APPARENT MAGNITUDE: +3.9 APPARENT DIMENSIONS: 36’

DISTANCE DETERMINATION Astronomers measure the proper motion and radial velocity of cluster stars, then use trigonometry to arrive at the distance.

AGE DETERMINATION Astronomers plot the colors and magnitudes of cluster stars on an H-R diagram to get an overall picture of the evolutionary states of the cluster stars. This, in turn, allows astronomers to constrain the age of the cluster.

NOTABLE FEATURES/FACTS • Omega Centauri contains at least one million stars. • Pinpointing the age of Omega Centauri is difficult because it appears to contain as many as 15 distinct stellar populations. Estimates range from 11 to 13 billion years old. • The presence of multiple populations of stars in Omega Centauri suggests the cluster may be a remnant core of a dwarf galaxy that was destroyed by the Milky Way billions of years ago. • Omega Centauri is the most massive globular star cluster in the Milky Way. Mass estimates range from about 2.5 million to 5 million times the mass of the Sun. • The average separation between stars in Omega Centauri is about 0.1 ly. • This object was originally misidentified as a faint star, hence its stellar designation. • Omega Centauri may contain an intermediate mass black hole. Measurement of stellar motions near the cluster center suggest the black hole could have a mass of up to 40,000 times the mass of the Sun. • Edmond Halley was the first astronomer of the modern era to describe Omega Centauri. He observed it in 1677. • Charles Messier never added this catalog to his catalog. Although it was certainly the type of object he would have noted, it was just too far south for him to see from Paris. SUMMER SP TARGET INFORMATION

MESSIER 3

BASIC INFORMATION OBJECT TYPE: Globular Cluster CONSTELLATION: Canes Venatici BEST VIEW: June DISCOVERY: Charles Messier, 1764 DISTANCE: 33,900 ly DIAMETER: 180 ly MASS: 450,000 MSUN APPARENT MAGNITUDE: +6.2 APPARENT DIMENSIONS: 18’

DISTANCE DETERMINATION Globular clusters contain many RR Lyrae stars, which are a type of standard candle. These stars vary in brightness, and the period of variation relates to the star’s luminosity. Astronomers can measure the period of variability, and then calculate the luminosity. Comparison of luminosity to apparent magnitude yields the distance.

MASS DETERMINATION The motion of stars in a cluster is dictated by gravity, which is a function of mass. Astronomers measure the motion of cluster stars, and then calculate the mass using dynamical models.

NOTABLE FEATURES/FACTS • M3 contains approximately 500,000 stars. Of these, about 275 are known to be variable stars; this is the most found in any globular cluster. • The combined energy of all of the cluster stars is about 300,000 times the energy of the Sun. • Half of the cluster’s mass is concentrated within 11 light years of the center. • The cluster is estimated to be approximately 12 billion years old, making it one of the oldest structures in our Galaxy. • M3 is moving toward Earth at a speed of about 140 km/s (310,000 mph). • Recent analyses suggest M3 likely contains two distinct populations of stars. The cluster probably underwent at least two rounds of star formation. • Many “blue straggler” stars have been observed in M3. These are stars that appear much younger than the other stars in the cluster, largely due to their blue color. The outer layers of these stars may have been stripped away during interactions with other cluster stars, resulting in their blue color. • William Herschel was first to resolve the cluster into individual stars (1784). • Messier added this object to his catalog on 5/3/1764. SUMMER SP TARGET INFORMATION

MESSIER 51

BASIC INFORMATION OBJECT TYPE: Galaxy CONSTELLATION: Canes Venatici BEST VIEW: June DISCOVERY: Charles Messier, 1773 DISTANCE: ~25 million ly DIAMETER: ~80,000 ly (visible disk @ 25 Mly) APPARENT MAGNITUDE: +8.4 APPARENT DIMENSIONS: 11’ x 7’

DISTANCE DETERMINATION Redshift: Using spectroscopy, astronomers measure the speed at which a galaxy is receding from us. This rate of recession is directly proportional to the distance (Hubble’s Law).

More current estimates are obtained by measuring the characteristics of several recent supernovae in M51.

Expanding Photosphere: Astronomers can determine the apparent angular expansion and speed of an exploding star’s photosphere. The two values can then be compared to obtain the distance.

Standard Candle: A relatively new method relies on standard characteristics of Type II-P supernovae. These core-collapse supernovae exhibit a characteristic plateau in their light curves. Astronomers have recently been able to calibrate the luminosity of this plateau. Measuring the apparent magnitude of the plateau yields a distance.

NOTABLE FEATURES/FACTS • M51 was the first galaxy in which spiral structure was detected. Lord Rosse made the observation in 1845 using his 72-inch “Leviathan” telescope at Birr Castle, Ireland. • Pierre Mechain discovered M51’s companion galaxy, NGC 5195, in 1781. • M51 and NGC 5195 are currently in the process of merging. This interaction has enhanced the spiral structure of M51 and triggered new star formation. • Dynamic models suggest M51 and NGC 5195 had their first close encounter about 500 million years ago. Some recent models suggest they had a second close encounter 50-100 million years ago. • M51’s mass is estimated at about 160 billion times the mass of the Sun. This translates to at least 250 billion stars. • The center of M51 contains a supermassive black hole. The upper limit of its mass is about two million times the mass of the Sun. • M51 is the brightest member of the M51 Group. • The Whirlpool Galaxy is moving away from us at a speed of about 460 km/s (about one million mph). • Messier added this object to his catalog on the night of 13 October 1773. SUMMER SP TARGET INFORMATION

MESSIER 5

BASIC INFORMATION OBJECT TYPE: Globular Cluster CONSTELLATION: Serpens BEST VIEW: Early July DISCOVERY: Gottfried Kirch, 1702 DISTANCE: 24,500 ly DIAMETER: 165 ly MASS: 834,000 MSUN APPARENT MAGNITUDE: +5.6 APPARENT DIMENSIONS: 23’ AGE: 12-13 billion years

DISTANCE DETERMINATION Astronomers measure the proper motion and radial velocity of cluster stars, then use trigonometry to arrive at the distance. Another method involves measuring the variation in brightness of RR Lyrae stars in the cluster. The period of variation is directly related to their luminosity. Comparing the luminosity to the apparent magnitude yields the distance.

NOTABLE FEATURES/FACTS • William Herschel was first to resolve M5 into individual stars. He made the observation in 1791, and he counted about 200 stars. • M5 contains at least 100,000 stars. Some estimates go as high as 500,000 stars. • Charles Messier cataloged this object on 23 May 1764. • Using the Hubble Space Telescope, astronomers have observed many blue stragglers in M5. These stars appear bluer than other stars of the same type and age, suggesting their structures have been altered by close interactions with neighbors in the cluster. • Recent observations with the ESO’s Very Large Telescope in Chile have shown that M5 is rotating, making it just one of a few globular clusters in which evidence of systemic rotation has been observed.

SUMMER SP TARGET INFORMATION

MESSIER 4

BASIC INFORMATION OBJECT TYPE: Globular Cluster CONSTELLATION: Scorpius BEST VIEW: July DISCOVERY: Philippe Loys de Chéseaux, 1746 DISTANCE: 5600 – 7200 ly DIAMETER: 75 ly MASS: 450,000 MSUN APPARENT MAGNITUDE: +5.6 APPARENT DIMENSIONS: 36’ AGE: 12-13 billion years

AGE DETERMINATION White dwarf stars in the cluster cool at a predictable rate, related to their age. Using an H-R diagram and stellar evolution models to analyze the evolutionary states of other cluster stars further constrains the age.

NOTABLE FEATURES/FACTS • M4 was the first globular cluster to be resolved into individual stars. Charles Messier made the observation and added the cluster to his catalog on 8 May 1764. • Extinction due to interstellar dust makes a distance to M4 difficult to determine, but at less than 7200 ly, it’s one of the nearest globulars to Earth. • M4 appears to have a “bar” structure made up of 11th magnitude stars across its center. • There appear to be two distinct populations of stars within the cluster. • In 1987, astronomers discovered a millisecond pulsar in M4. It rotates over 300 times per second. • In 1995, the Hubble Space Telescope observed white dwarf stars in M4. These stars are 12.7 billion years old, making them some of the oldest stars in the Milky Way. • In 2003, astronomers identified a possible planet orbiting a white dwarf star in M4. The planet is 2.5 times the mass of Jupiter. SUMMER SP TARGET INFORMATION

MESSIER 13

BASIC INFORMATION OBJECT TYPE: Globular Cluster CONSTELLATION: Hercules BEST VIEW: Late July DISCOVERY: Edmond Halley, 1714 DISTANCE: 25,100 ly DIAMETER: 145 ly APPARENT MAGNITUDE: +5.8 APPARENT DIMENSIONS: 20’

DISTANCE DETERMINATION Globular clusters contain many RR Lyrae stars, which are a type of standard candle. These stars vary in brightness, and the period of variation relates to the star’s luminosity. Comparison of luminosity to apparent magnitude yields the distance.

NOTABLE FEATURES/FACTS • M13 contains several hundred thousand stars. Some estimates even go as high as one million. • The cluster is estimated to be 12 to 13 billion years old, making it one of the oldest structures in our Galaxy. • M13 contains at least two distinct populations of stars, indicating that the stars formed during multiple episodes. • Many “blue straggler” stars have been observed in M13. These are stars that appear much younger than the other stars in the cluster, largely due to their blue color. The outer layers of these stars may have been stripped away during interactions with other cluster stars, resulting in their blue color. • In 1974, the Arecibo radio telescope was used to send a signal to M13. The signal contained information including descriptions of DNA, human physiology, and our Solar System. If anyone is in M13 to receive it, we can expect a reply in about 50,000 years. • Messier added this object to his catalog on 1 June 1764.

SUMMER SP TARGET INFORMATION

MESSIER 6 (The Butterfly Cluster)

BASIC INFORMATION OBJECT TYPE: Open Cluster CONSTELLATION: Scorpius BEST VIEW: August DISCOVERY: Giovanni Batista Hodierna, c. 1654 DISTANCE: 1600 ly DIAMETER: 12 – 25 ly APPARENT MAGNITUDE: +4.2 APPARENT DIMENSIONS: 25’ – 54’ AGE: 50 – 100 million years

DISTANCE DETERMINATION The colors and magnitudes of cluster members are plotted on an H-R diagram. Specific features, such as the location of the main sequence, have known theoretical positions on the plot. Comparison of the observed position of these features to the theoretical position can yield the distance.

NOTABLE FEATURES/FACTS • There is some speculation that Ptolemy observed this cluster as early as the 2nd Century A.D. • The “Butterfly Cluster” nickname is attributed to Robert Burnham, Jr., who described the cluster as a “charming group whose arrangement suggests the outline of a butterfly with open wings.” This arrangement is best seen in binoculars. • M6 contains about 80 stars, but some estimate go as high as several hundred. • The brightest star in M6 is an orange supergiant known as BM Scorpii. It is a semi-regular variable whose brightness varies over about 1.5 magnitudes. • Charles Messier cataloged this object on the night of 23 May 1764.

SUMMER SP TARGET INFORMATION

MESSIER 7 (Ptolemy’s Cluster)

BASIC INFORMATION OBJECT TYPE: Open Cluster CONSTELLATION: Scorpius BEST VIEW: August DISCOVERY: Claudius Ptolemy, 130 A.D. DISTANCE: 900 – 1000 ly DIAMETER: 20 – 25 ly APPARENT MAGNITUDE: +3.3 APPARENT DIMENSIONS: 80’ AGE: ~220 million years

NOTABLE FEATURES/FACTS • Claudius Ptolemy was first to catalog this cluster, listing it as a “nebula following the sting of Scorpius” in his Almagest. • M7 contains about 80 stars. • The brightest star in M7 is a type G8 yellow giant of magnitude 5.6. • Charles Messier cataloged this object on the night of 23 May 1764.

SUMMER SP TARGET INFORMATION

MESSIER 8 (THE LAGOON NEBULA)

BASIC INFORMATION OBJECT TYPE: Star Forming Region CONSTELLATION: Sagittarius BEST VIEW: August DISCOVERY: Hodierna, 1654 (nebula) Le Gentil, 1747 (nebula & cluster) DISTANCE: 4000 – 6000 ly DIAMETER: Approximately 130 ly x 60 ly APPARENT MAGNITUDE: +6.0 APPARENT DIMENSIONS: 90’x40’

DISTANCE DETERMINATION Astronomers can identify clusters of stars associated with the nebula, and then plot their colors and magnitudes on an H-R Diagram. Comparing the apparent magnitudes of cluster stars with known absolute magnitudes for given points on the diagram can yield the distance. This is very difficult because many stars have some level of obscuration by the nebula. Thus, the distance to M8 is uncertain.

AGE DETERMINATION The presence of features associated with pre-main-sequence stars provides an upper limit to the age. Astronomers can also plot the colors and magnitudes of nebula stars on an H-R Diagram to get an overall picture of the evolutionary states of those stars and further constrain the age.

NOTABLE FEATURES/FACTS • M8 is embedded in a large molecular cloud, a huge complex of cool gas that has the potential to produce thousands of stars. • The energy emitted by the young stars within M8 is responsible for the visibility of the nebula. Starlight triggers emission in gaseous regions, giving the nebula its distinctive glow. • Most observers will see the nebula as mostly gray in color, but some may pick out a green tint. This is due to OIII emission (doubly-ionized oxygen). • The open cluster NGC 6530 is part of the same molecular cloud as M8. The stars of the cluster appear slightly in front of the nebula from our perspective. NGC 6530 contains several hundred stars and is about two million years old. • Several Bok Globules are present in M8. These are collapsing protostellar clouds that appear as dark spots within the nebula. • At the center of M8 lies the Hourglass Nebula. This is the brightest part of M8 and is an active site of star formation. • The Hubble Space Telescope has detected several large, funnel-shaped clouds associated with strong stellar winds in M8. The clouds are 0.5 ly long. • Messier added this object to his catalog on 23 May 1764. SUMMER SP TARGET INFORMATION

MESSIER 20 (THE TRIFID NEBULA)

BASIC INFORMATION OBJECT TYPE: Star Forming Region CONSTELLATION: Sagittarius BEST VIEW: August DISCOVERY: Messier, 1764 DISTANCE: ~5200 ly DIAMETER: 42 ly APPARENT MAGNITUDE: +9.0 APPARENT DIMENSIONS: 11’

NOTABLE FEATURES/FACTS • The name “Trifid” refers to the three-lobed appearance of the nebula. The lobes are divided by a dark nebula, known as Barnard 85. • A cluster of young stars is embedded in the central region of the nebula. These stars are estimated to be as young as 300,000 years old. • Most of the emission in the heart of the nebula is attributed to a single O- class star. • M20 contains both an emission and a reflection nebula. The central star has sufficient energy to excite atoms in the gas nearby, but it lacks the energy to prompt emission in the outskirts of the nebula. The outer portion of the nebula glows instead by reflected light. • Messier added this object to his catalog on the night of 5 June 1764. SUMMER SP TARGET INFORMATION

MESSIER 17 (THE SWAN NEBULA)

BASIC INFORMATION OBJECT TYPE: Star Forming Region CONSTELLATION: Sagittarius BEST VIEW: Late August DISCOVERY: Philippe Loys deCheseaux, 1745 DISTANCE: 5200 – 6800 ly DIAMETER: 15 – 20 ly APPARENT MAGNITUDE: +6.0 APPARENT DIMENSIONS: 11’

DISTANCE DETERMINATION Astronomers can identify clusters of stars associated with the nebula, and then plot their colors and magnitudes on an H-R Diagram. Comparing the apparent magnitudes of cluster stars with known absolute magnitudes for given points on the diagram can yield the distance. Observations are made in infrared light because the nebula obscures the cluster stars. This heavy obscuration makes the distance to M17 uncertain.

AGE DETERMINATION Astronomers plot the colors and magnitudes of nebula stars on an H-R Diagram to get an overall picture of the evolutionary states of the nebula stars and constrain the age. As with distance measurements, the dense nebular material makes these observations difficult and creates some uncertainty in the age value.

NOTABLE FEATURES/FACTS • M17 is known by many names: the Swan Nebula, the Omega Nebula, the Checkmark Nebula, the Lobster Nebula, and the Horseshoe Nebula. • The brightest part of M17 is lit from within by about a dozen O-class stars. These stars are part of a larger cluster of up to 800 stars embedded in the heart of the nebula. • The central cluster is estimated to be between one and three million years old. • As many as 10,000 stars have been associated with the entire nebula, which spans about 40 light years. • Three-dimensional models suggest the nebula is bowl shaped, and we see this structure edge-on from Earth. • Messier added this object to his catalog on the night of 3 June 1764. SUMMER SP TARGET INFORMATION

MESSIER 22

BASIC INFORMATION OBJECT TYPE: Globular Cluster CONSTELLATION: Sagittarius BEST VIEW: Late August DISCOVERY: Abraham Ihle, 1665 DISTANCE: 10,400 ly DIAMETER: 97 ly APPARENT MAGNITUDE: +5.1 APPARENT DIMENSIONS: 32’

DISTANCE DETERMINATION Globular clusters contain many RR Lyrae stars, which are a type of standard candle. These stars vary in brightness, and the period of variation relates to the star’s luminosity. Comparison of luminosity to apparent magnitude yields the distance.

NOTABLE FEATURES/FACTS • M22 was likely the first globular cluster ever discovered. We now know of about 150 such objects in the Milky Way. • M22 contains at least 70,000 stars and most likely several hundred thousand. Some estimates even go as high as half a million. • The cluster is estimated to be 12 to 13 billion years old, making it one of the oldest structures in our Galaxy. • M22 is one of only four globular clusters in which a planetary nebula has been detected. • Using the Very Large Array, astronomers have detected two stellar-mass black holes in M22, the first ever detected in a globular cluster. • The Hubble Space Telescope has detected several free-floating, planet-sized objects in M22. These objects were detected using microlensing techniques. • M22 is moving away from us at a rate of about 150 km/s (about 335,000 mph). • In 2019, astronomers detected the remains of a nova that occurred about 2000 years ago. The event may have been recorded by Chinese astronomers in 48 BCE. • Messier added this object to his catalog on the night of 5 June 1764. SUMMER SP TARGET INFORMATION

EPSILON LYRAE (e LYR or “THE DOUBLE DOUBLE”)

BASIC INFORMATION OBJECT TYPE: Multiple Star System CONSTELLATION: Lyra BEST VIEW: Late August DISCOVERY: F.G.W. Struve, 1831 DISTANCE: 163 ly BINARY SEPARATION (e) : 208” (>10,000 AU) BINARY SEPARATION (e1): 2.8” (235 AU avg.) BINARY SEPARATION (e2): 2.2” (145 AU avg.) ORBITAL PERIOD (e): At least 400,000 years ORBITAL PERIOD (e1): ~1800 years ORBITAL PERIOD (e2): ~724 years APPARENT MAGNITUDE: 3.8

DISTANCE DETERMINATION After measuring the shift in position of the star relative to background stars as Earth orbits the Sun, simple trigonometry can yield the distance. The Hipparcos satellite was launched in 1989 to create a comprehensive catalog of trigonometric parallax measurements from space. Parallax measurements of binary systems are notoriously difficult to obtain due to the respective motions of the two components. So, the Hipparcos measurement quoted above is considered an estimate.

NOTABLE FEATURES/FACTS • Epsilon Lyrae is visible to the naked eye, but its multiple nature was not discovered until 1831. Friedrich Georg Wilhelm von Struve made the discovery. Struve is the great-grandfather of Otto Struve, the first director of McDonald Observatory. • This system consists of at least four A-class main sequence stars. At low resolution, two stars are evident. When using higher resolution equipment, each of the two components can be split into two. • Although there is limited data, the best estimates suggest this system is just 800 million years old. • The two pairs are separated by at least 10,000 AU (~930 billion miles). From one of the pairs, an observer would see the other appear as bright as a quarter moon, with about a degree separating the members. • Astronomers are still trying to figure out how systems like this form and evolve. The gravity of nearby stars and Galactic tides will probably cause the system to separate over time.

SUMMER SP TARGET INFORMATION

MESSIER 57 (THE RING NEBULA)

BASIC INFORMATION OBJECT TYPE: Planetary Nebula CONSTELLATION: Lyra BEST VIEW: Late August DISCOVERY: Charles Messier, 1779* DISTANCE: 2300 ly DIAMETER: 0.9 ly (bright ring), 2.5 ly (IR halo) APPARENT MAGNITUDE: +8.8 (nebula), +15.7 (central star) APPARENT DIMENSIONS: 1.4’ x 1.0’

DISTANCE DETERMINATION The distances to most planetary nebulae are very poorly known. A variety of methods can be used, providing mixed results.

Trigonometric parallax is considered one of the most direct methods of measuring the distance to a planetary nebula. As Earth orbits the Sun, the position of nearby objects will shift relative to more distant objects. Measuring this shift and applying simple trigonometry can yield the distance to the nearby object. For planetary nebulae, the parallax shift of the central star is measured. However, atmospheric effects make it difficult to get accurate measurements from the ground for objects more than a few hundred light years away. Space-based measurements eliminate this problem, but few such measurements of planetary nebulae have been made so far. The adopted value above is based on ground-based parallax measurements, but it is consistent with values obtained using other methods.

NOTABLE FEATURES/FACTS • *Antoine Darquier de Pellepoix has traditionally been credited with discovery of M57. However, a recent examination of Messier’s notes and correspondence between the two astronomers suggests Messier was first to note the nebula. • The Ring Nebula is the remnant of a dying star that was once slightly larger than the Sun. When the Sun exhausts its fuel in about five billion years, it will produce a similar structure. • Three-dimensional modeling and Hubble Space Telescope imagery suggest the nebula is a prolate spheroid (football-shaped), with a torus (donut shape) of material at its equator. We are viewing the nebula from one end of the football, and the bright visible ring is the torus. • The nebula is estimated to be about 7000 years old. This is based on the current expansion rate of approximately 25 km/s (56,000 mph) and models of the structural evolution of planetary nebulae. • The central star is transitioning from a giant star to a white dwarf. Its current temperature is about 120,000 K (215,000 °F), and it will cool over billions of years to become a black dwarf. • The energy from the central star causes the nebular material to glow. Colors seen in photographs correspond to different elements in the gas. • Messier added this object to his catalog on the night of 31 January 1779. SUMMER SP TARGET INFORMATION

MESSIER 11 (THE WILD DUCK CLUSTER)

BASIC INFORMATION OBJECT TYPE: Open Cluster CONSTELLATION: Scutum BEST VIEW: September DISCOVERY: Gottfried Kirch, 1681 DISTANCE: Approx. 6000 ly DIAMETER: Approx. 50 ly APPARENT MAGNITUDE: +6.3 APPARENT DIMENSIONS: 14’ AGE: Approx. 220 million years

NOTABLE FEATURES/FACTS • The cluster is nicknamed “The Wild Duck” because some of its brighter stars may look like a flock of flying ducks in small telescopes. Your mileage may vary. • Messier 11 contains about 2900 stars. • The brightest star in the field will appear to be part of the cluster, but it is really a foreground star. • Charles Messier added this object to his catalog on 30 May 1764. SUMMER SP TARGET INFORMATION

ALBIREO (b CYG)

BASIC INFORMATION OBJECT TYPE: Double Star CONSTELLATION: Cygnus BEST VIEW: Late September DISCOVERY: Known to Ancients DISTANCE: Albireo A: 328 ly Albireo B: 389 ly BINARY SEPARATION: 35” (3800 AU) APPARENT MAGNITUDE: 3.1

NOTABLE FEATURES/FACTS • William Herschel discovered Albireo’s dual nature in 1779. In 1832, F.G.W. Struve added Albireo to his catalog of double stars. Struve is the great- grandfather of Otto Struve, the first director of McDonald Observatory. • Albireo displays a striking contrast in color. The brighter A star is a golden color, while the fainter B star is blue. • The color difference between the stars reveals a difference in temperature. The gold-colored A star is cooler than the blue B star. • The B star is about 200 times brighter than the Sun, while the A star is over a thousand times brighter! • Astronomers long considered Albireo to be a possible binary star system. However, proper motion measurements from the Gaia satellite indicate the two stars are moving at directions and speeds inconsistent with orbital motion. Albireo is not a binary system. • Albireo A, however, is itself a true spectroscopic binary. The separation of the A pair is less than one arcsecond, and the orbital period is about 200 years. SUMMER SP TARGET INFORMATION

MESSIER 27 (THE DUMBBELL NEBULA)

BASIC INFORMATION OBJECT TYPE: Planetary Nebula CONSTELLATION: Vulpecula BEST VIEW: Early October DISCOVERY: Charles Messier, 1764 DISTANCE: 1360 ly DIAMETER: 3 ly APPARENT MAGNITUDE: +7.4 APPARENT DIMENSIONS: 8.0’ x 5.7’

DISTANCE DETERMINATION The distances to most planetary nebulae are very poorly known. A variety of methods can be used, providing mixed results. For example, distance estimates for M27 using various methods range from about 500 to 1370 light years!

Trigonometric parallax is considered one of the most direct methods of measuring the distance to a planetary nebula. As Earth orbits the Sun, the position of nearby objects will shift relative to more distant objects. Measuring this shift and applying simple trigonometry can yield the distance to the nearby object. For planetary nebulae, the parallax shift of the central star is measured. However, the planetary nebulae are too far away for ground-based telescopes to detect their parallax well, so the measurements need to be made from space. The adopted value above is based on parallax measurements obtained with the Hubble Space Telescope.

NOTABLE FEATURES/FACTS • M27 is also known as the “Apple Core Nebula.” • The “Dumbbell” name is attributed to John Herschel. • M27 was the first planetary nebula to be discovered. • Three-dimensional modeling suggests the nebula is a prolate spheroid (football-shaped) and we are viewing it along its equatorial plane. If viewed from a pole, it may look like M57 (the Ring Nebula). • The central star is estimated to have about half the mass of the Sun packed into a volume about half the size of Jupiter. • The central star’s temperature is about 85,000 K (150,000 °F). • Messier added this object to his catalog on the night of 12 July 1764. SUMMER SP TARGET INFORMATION

MESSIER 15

BASIC INFORMATION OBJECT TYPE: Globular Cluster CONSTELLATION: Pegasus BEST VIEW: Late October DISCOVERY: Jean-Dominique Maraldi, 1746 DISTANCE: 33,600 ly DIAMETER: 175 ly APPARENT MAGNITUDE: +6.2 APPARENT DIMENSIONS: 18’

DISTANCE DETERMINATION Globular clusters contain many RR Lyrae stars, which are a type of standard candle. These stars vary in brightness, and the period of variation relates to the star’s luminosity. Comparison of luminosity to apparent magnitude yields the distance.

NOTABLE FEATURES/FACTS • M15 contains several hundred thousand stars. • The cluster is estimated to be approximately 13 billion years old, making it one of the oldest structures in our Galaxy. • The total energy output of M15’s stars is 360,000 times the energy of the Sun. • M15 is the most dense globular cluster. Half of its mass is contained within 10 ly of its center. This is probably due to core collapse: stars have settled near the center due to their gravitational influence on one another. • Some astronomers suspect there may be an intermediate-mass black hole at the center of M15. Recent studies, however, have found no evidence of one. • M15 contains Pease 1, the first planetary nebula ever detected in a globular cluster. To date, only a handful of planetaries have been discovered in globulars. • In 2016, astronomers using the Fermi Large Area Telescope reported significant gamma ray emission from M15. The source of the gamma rays is unknown, but may come from a population of millisecond pulsars or from jets emanating from intermediate mass black holes. • Messier added this object to his catalog on 3 June 1764.