WINTER 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. 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 • 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. WINTER SP TARGET INFORMATION NGC 7009 BASIC INFORMATION OBJECT TYPE: Planetary Nebula CONSTELLATION: Aquarius BEST VIEW: Early November DISCOVERY: William Herschel, 1782 DISTANCE: 2000 - 4000 ly DIAMETER: 0.4 - 0.8 ly APPARENT MAGNITUDE: +8.0 APPARENT DIMENSIONS: 41” x 35” 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: Trigonometric Parallax – Measure the apparent shift of the central star relative to background stars as Earth orbits the Sun or as a spacecraft orbits Earth. Trigonometry yields the distance. Statistical Analyses – Correlate measurable physical properties of the nebula with distance. This requires making some big assumptions, like 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 • This object’s nickname comes from its resemblance to the shape of the planet Saturn. It was first described this way by Lord Rosse in the 1840’s. • The Saturn 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. • NGC 7009 has two distinct “handles” or “ansae.” These are jets emanating from the central white dwarf. They are responsible for the Saturn-like appearance of the nebula. • The energy from the central star causes the nebular material to glow. • Green color visible in the nebula is the result of ionization of oxygen atoms in the cloud. WINTER SP TARGET INFORMATION 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. WINTER SP TARGET INFORMATION 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 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. 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 • 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. WINTER 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. WINTER SP TARGET INFORMATION NGC 253 (THE SCULPTOR GALAXY) BASIC INFORMATION OBJECT TYPE: Galaxy CONSTELLATION: Sculptor BEST VIEW: December 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.