Fall Sp Target Information

Home , 25 Scorpii, 30 Vulpeculae, 31 Vulpeculae, 32 Vulpeculae, 4 Scorpii, Butterfly Cluster

FALL 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.

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 • 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.

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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.

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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.

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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 hunded 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. FALL 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. FALL 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. FALL 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’

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. FALL 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.

FALL 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. FALL 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. FALL 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) ORBITAL PERIOD: 75,000+ yr. 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. FALL 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. FALL 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’

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.

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NGC 7662

BASIC INFORMATION OBJECT TYPE: Planetary Nebula CONSTELLATION: Andromeda BEST VIEW: Late November DISCOVERY: William Herschel, 1784 DISTANCE: 1800 – 6400 ly DIAMETER: 0.3 – 1.1 ly APPARENT MAGNITUDE: +8.6 APPARENT DIMENSIONS: 37”

DISTANCE DETERMINATION The distances to most planetary nebulae are very poorly known. A variety of methods can be used, providing mixed results. The range adopted above represents a variety of recent studies, using methods including:

Statistical Analyses – Correlate measurable physical properties of the nebula with distance. This requires making some big assumptions, like that all planetary nebulae have similar properties.

Expansion Velocity – Measure the rate at which the nebula is expanding in physical units and angular measure. Trigonometry yields the distance.

NOTABLE FEATURES/FACTS • The Blue Snowball is the remnant of a dying star that was about the same size as the Sun. When the Sun exhausts its fuel in about five billion years, it will produce a similar structure. • The central star is transitioning to a white dwarf. Its temperature is estimated at 80,000K to 110,000K (approx. 140,000° – 200,000° F). The energy from this central star causes the nebular material to glow. • The distinct blue color of the nebula is due to emission from ionized oxygen atoms. • Nebular material is expanding away from the central star at speeds of up to 130,000 miles per hour.

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MESSIER 52

BASIC INFORMATION OBJECT TYPE: Open Cluster CONSTELLATION: Cassiopeia BEST VIEW: December DISCOVERY: Charles Messier, 1774 DISTANCE: ~5000 ly DIAMETER: 19 ly APPARENT MAGNITUDE: +7.3 APPARENT DIMENSIONS: 13’ AGE: 50 million years

NOTABLE FEATURES/FACTS • M52 has at least 200 confirmed members. Mass estimates suggest it could have as many as 2000 stars. • The stellar population of M52 appears to have formed in multiple phases over a span of about 50 million years, with low mass stars forming early and high mass stars forming more recently. • Charles Messier cataloged this object on the night of 7 September 1774. He spotted it while observing a comet that passed near the cluster’s position on the sky. FALL SP TARGET INFORMATION

MESSIER 31 (THE ANDROMEDA GALAXY)

BASIC INFORMATION OBJECT TYPE: Galaxy CONSTELLATION: Andromeda BEST VIEW: December DISCOVERY: Abd al-Rahman al-Sufi, 964 DISTANCE: 2.5 million ly DIAMETER: ~250,000 ly* APPARENT MAGNITUDE: +3.4 APPARENT DIMENSIONS: 178’ x 63’ (3° x 1°)

*This value represents the total diameter of the disk, based on multi-wavelength measurements. The bright visible disk is closer to 120,000 ly.

DISTANCE DETERMINATION Astronomers use Cepheid variables to gauge the distance to galaxies. 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.

NOTABLE FEATURES/FACTS • M31 is the nearest spiral galaxy to the Milky Way. These two galaxies are the largest members of the Local Group. • Two satellite galaxies, M32 and M110, are usually visible in telescopic views of M31. • M32 may be the remnant core of a large galaxy that collided with M31 about 2 billion years ago. • M31 contains about 110 billion times the mass of the Sun, or about one trillion stars. • M31 has a larger diameter and more visible mass than the Milky Way. However, the Milky Way may contain more dark matter. • M31 and the Milky Way are rushing toward each other and will collide in about 4.5 billion years. • The center of M31 contains a supermassive black hole of approximately 140 million solar masses. • A microlensing event detected in M31 in 1999 may have been caused by passage of a planet in front of a star. If so, this would be the first planet detected outside the Milky Way. • In 2015, astronomers using the Hubble Space Telescope detected evidence of a huge, gaseous halo surrounding M31. It stretches over one million light years from M31, or almost halfway to the Milky Way. • Messier added this object to his catalog on the night of 3 August 1764. FALL SP TARGET INFORMATION

NGC 253 (THE SCULPTOR GALAXY)

BASIC INFORMATION OBJECT TYPE: Galaxy DISCOVERY: Caroline Herschel, 1783 DISTANCE: ~11 million ly DIAMETER: ~87,000 ly APPARENT MAGNITUDE: +8.0 APPARENT DIMENSIONS: 27’ x 6’

DISTANCE DETERMINATION Two methods are commonly used to determine the distance to NGC 253. 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.

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.

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 • NGC 253 is also known as the “Silver Coin” or “Silver Dollar” galaxy. • This galaxy is the nearest “starburst” galaxy to the Milky Way. It is undergoing a bout of intense star formation, possibly sparked by an interaction with a dwarf galaxy about 200 million years ago. • NGC 253 is the central and brightest member of the Sculptor Group of galaxies, one of the nearest galaxy clusters to our own Local Group. • In 2015/2016, astronomers discovered that NGC 253 is in the process of consuming a faint dwarf galaxy. The tiny companion was first observed by an Australian amateur astronomer and has since been investigated by two independent professional groups. • When Caroline Herschel discovered this galaxy, she was using a 4-inch telescope that magnified her view by at most 30x.

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M76 (THE LITTLE DUMBBELL NEBULA)

BASIC INFORMATION OBJECT TYPE: Planetary Nebula CONSTELLATION: Perseus BEST VIEW: Late December DISCOVERY: Pierre Mechain, 1780 DISTANCE: ~2400 ly DIAMETER: 0.8 ly (brightest part at 2400 ly) APPARENT MAGNITUDE: +10.1 (nebula), +16.6 (central star) APPARENT DIMENSIONS: 2.7’ x 1.8’

DISTANCE DETERMINATION The distances to most planetary nebulae are very poorly known. A variety of methods can be used, providing mixed results. For instance, the range of published distances for M76 is from 1700 to 15,000 ly!

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 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. Astronomers measure physical properties (e.g. mass, temperature) of lots of planetary nebulae in the LMC, and then 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 • Early observers assumed this object was actually two separate emission nebulae. This prompted William Herschel to give M76 two entries in his New General Catalog (NGC 650 and NGC 651). • This object’s nickname comes from its resemblance to M27, the Dumbbell Nebula. Other nicknames include: the Barbell Nebula and the Cork Nebula. • The Little Dumbbell 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. • M76 contains an evolved central star surrounded by a dusty torus of material. Its two lobes are “bubbles” of gas being blown away from the central star by a stellar “superwind.” • The central star is actually an optical double. The apparent companion is also itself a double star. Recent observations, however, suggest the central star and companion are not physically related. • The central star is transitioning from a giant star to a white dwarf. • The energy from the central star causes the nebular material to glow. • Messier added this object to his catalog on the night of 21 October 1780. FALL SP TARGET INFORMATION

NGC 457

BASIC INFORMATION OBJECT TYPE: Open Cluster CONSTELLATION: Cassiopeia BEST VIEW: January DISCOVERY: William Herschel, 1787 DISTANCE: Approximately 8000 ly DIAMETER: 30 ly APPARENT MAGNITUDE: +6.4 APPARENT DIMENSIONS: 13’ AGE: ~21 million years

NOTABLE FEATURES/FACTS • NGC 457 is known by a variety of common names, due to its distinctive shape. Among its aliases: the Owl Cluster, the E.T. Cluster, the Kachina Doll and the Skiing Cluster. • This cluster contains approximately 150 stars.

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THE PERSEUS DOUBLE CLUSTER (NGC 869 & NGC 884)

BASIC INFORMATION OBJECT TYPE: Open Clusters CONSTELLATION: Perseus BEST VIEW: January DISCOVERY: Hipparchus, 130 B.C. DISTANCE: Approx. 7000 ly DIAMETER: ~200 ly APPARENT MAGNITUDE: 3.7 (869), 3.8 (884) APPARENT DIMENSIONS: 100’ AGE: ~13 million years

NOTABLE FEATURES/FACTS • As many as 5000 cluster members have been identified within 10’ of the cluster cores. The region may contain at least 20,000 cluster members. A few hundred of the brightest stars of each cluster are visible in the telescope. • The two clusters are separated from one another by less than 300 light years. • The cores of the two clusters are nearly identical in their stellar composition and age. They differ only in their mass and spatial extent; NGC 869 is more dense. • The two clusters likely formed at the same time, from the same nebula. There is also speculation that the two clusters are a dual core to a larger stellar association in the direction of Perseus. FALL SP TARGET INFORMATION

MESSIER 45 (THE PLEIADES)

BASIC INFORMATION OBJECT TYPE: Open Cluster CONSTELLATION: Taurus BEST VIEW: Late January DISCOVERY: Known in Antiquity DISTANCE: 440 ly DIAMETER: 86 ly APPARENT MAGNITUDE: +1.6 APPARENT DIMENSIONS: 110’ AGE: ~100 million years

DISTANCE DETERMINATION Trigonometric parallax: 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.

MS Cluster Fitting: 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 • The earliest reference to the Pleiades is by Homer in 750 B.C. • M45 is also known as “The Seven Sisters,” referring to the seven daughters of Atlas in Greek mythology. In Japan, the cluster is known as “Subaru.” • The Pleiades contains at least 1000 stars. • Nebulosity is evident in photographs and in views through large telescopes. The nebulosity is due to reflection off dust grains in a cloud through which the cluster is traveling. It is not a remnant of the cloud from which the stars formed, as previously thought. • Charles Messier cataloged this object on the night of 4 March 1769. There is speculation he added this object simply to boost the number of objects in his catalog to exceed that of his contemporary, Nicholas Louis de la Caille. • Many Pleiades stars appear blue, typical of hot, young stars. • M45 serves as one of the first rungs in the cosmic distance ladder. Accurate, direct determination of its distance allows calibration of other distance determination methods for more distant objects.