SPRING 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. 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 • 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. • Astronomers haVe detected large amounts of dust surrounding one Pleiades member. The dust is thought to be the result of large collisions among bodies in a still forming solar system. SPRING SP TARGET INFORMATION NGC 1535 (CLEOPATRA’S EYE) BASIC INFORMATION OBJECT TYPE: Planetary Nebula CONSTELLATION: Eridanus BEST VIEW: February DISCOVERY: William Herschel, 1785 DISTANCE: 3960 – 7520 ly DIAMETER: Approximately 1 ly APPARENT MAGNITUDE: +10.5 APPARENT DIMENSIONS: 0.6’ 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 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 • Cleopatra’s Eye 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. 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. • The density of the nebular material indicates it is a relatiVely young object. The planetary nebula likely began forming just a few thousand years ago. • Upon discoVering the star in 1845, J.R. Hind described it as looking “like a drop of blood on a black field.” • R Lep is currently about 500 times the size of the Sun. If placed at the center of our solar system, it would engulf the Earth and extend almost halfway to Jupiter. SPRING SP TARGET INFORMATION HIND’S CRIMSON STAR (R LEP) BASIC INFORMATION OBJECT TYPE: Carbon Star CONSTELLATION : Lepus BEST VIEW: February DISCOVERY: J. R. Hind, 1845 DISTANCE: 1360 ly MASS: 2.5-5 MSOLAR APPARENT MAGNITUDE: +5.5 to +11.7* *Magnitude estimates are based on the star’s 40-year cycle. 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 Gaia satellite was launched in 2013 to create a comprehensiVe catalog of trigonometric parallax measurements from space. The distance quoted aboVe is from this catalog. MASS DETERMINATION The mass quoted aboVe is an estimate based on the aVerage masses of this kind of star. A direct mass measurement is unaVailable. NOTABLE FEATURES/FACTS • R Lep is a Mira-type variable star. Its brightness varies by about four magnitudes (approx. +7 to +11) oVer a period of about 14.5 months. This Variation is due to stellar pulsations common at the end of a star’s life. • Typically, R Lep is visible only with optical aid, but it also has a longer period of variation oVer about 40 years. At its brightest during this cycle, R Lep is just barely visible to the unaided eye. This variation is likely due to ejection of stellar material. • This star is in its final stages of eVolution. It is currently expelling its outer layers into space and will ultimately form a planetary nebula. • ConVection has dredged up large amounts of carbon from the core to the outer layers of the star. This carbon absorbs blue light, giVing the star its distinct crimson color. • The redness of the star varies with its brightness. It looks most red when at its dimmest. • Upon discoVering the star in 1845, J.R. Hind described it as looking “like a drop of blood on a black field.” • R Lep is currently about 500 times the size of the Sun. If placed at the center of our solar system, it would engulf the Earth and extend almost halfway to Jupiter. SPRING SP TARGET INFORMATION MESSIER 79 BASIC INFORMATION OBJECT TYPE: Globular Cluster CONSTELLATION: Lepus BEST VIEW: Late February DISCOVERY: Pierre Mechain, 1780 DISTANCE: 42,000 ly DIAMETER: 115 ly MASS: 200,000 MSUN APPARENT MAGNITUDE: +8.2 APPARENT DIMENSIONS: 9.6’ AGE: 10-12 billion years DISTANCE DETERMINATION Astronomers measure 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. 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. 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 • William Herschel was first to resolVe M79 into indiVidual stars. He made the obserVation in 1784. • M79 contains about 100,000 stars. • This cluster may not be natiVe to our galaxy. The Milky Way is currently in the process of “eating” the Canis Major Dwarf Galaxy. M79 may haVe been captured during this ongoing eVent. • M79 may contain an intermediate mass black hole. If it exists, the black hole could be up to 3000 times as massiVe as the Sun. • Charles Messier cataloged this object on 17 December 1780. SPRING SP TARGET INFORMATION MESSIER 1 (THE CRAB NEBULA) BASIC INFORMATION OBJECT TYPE: SupernoVa Remnant CONSTELLATION: Taurus BEST VIEW: Late February DISCOVERY: John BeVis, 1731 DISTANCE: 6500 ly DIAMETER: 11 ly MASS: 4.6 MSUN APPARENT MAGNITUDE: +8.4 APPARENT DIMENSIONS: 6’x4’ DISTANCE DETERMINATION Using spectroscopy, astronomers directly measure the absolute expansion rate of the nebula. Comparing images of the nebula oVer time proVides a measure of the apparent angular expansion. This value directly relates to the distance: for a giVen absolute expansion, a nearby object will haVe a higher apparent angular expansion than a distant one. AGE DETERMINATION The expansion rate of the nebula is measured directly using spectroscopy. From this, astronomers work backward to the time at which the nebula’s extent would haVe been zero. This yields the age of the nebula, and thus, the date of the supernoVa. NOTABLE FEATURES/FACTS • The Crab Nebula is the remnant of a Type II supernoVa that was first obserVed on 4 July 1054. According to Chinese records, this “guest star” was Visible to the unaided eye for about two years; it was visible in broad daylight for seVeral weeks. • The Crab Nebula is currently expanding at a rate of about 1500 km/s (3.4 million mph). • Three-dimensional models suggest the nebula is a prolate spheroid, or shaped somewhat like a football. • At the center of the nebula is the Crab Pulsar, a neutron star, which is spinning at a rate of about 30 times a second. • The Crab Pulsar is about 30 km (19 mi.) across, and its mass is estimated to be a little greater than the mass of the Sun. • Across all waVelengths, the Crab Pulsar is about 100,000 times more energetic than the Sun. Energy from the pulsar plays a significant role in causing the nebula to glow. • The Crab Pulsar’s progenitor star was at least eight times the mass of the sun. • In the 1840’s, Lord Rosse made a drawing of the nebula that he thought looked like a crab, so he called the object the “Crab Nebula.” • Messier added this object to his catalog on 9/12/1758.