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 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. The distance quoted above is from this catalog. 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. 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.