Midterm Review 2.doc

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University of Toronto St. George
Astronomy & Astrophysics
Michael Reid

MIDTERM 2 1. What is the diameter of the Milky Way galaxy and roughly how many stars does it contain?  100, 000 light years across (diameter across)  1000 light years thick (up and down)  100 to 400 billion stars 2. How far does the Sun orbit from the center of the Milky Way galaxy?  Our Sun is located about 28, 000 light years from the galactic centre 3. How much energy would a solar panel absorb if it were located 10 AU from the Sun compared to the amount it would absorb if it were 1 AU from the Sun? 4. In a given image, which star has the highest luminosity? 5. Which types of binary star systems are most commonly observed (e.g. eclipsing, spectroscopic, visual) and why?  Visual – we can see both members of the system; this only works for nearby binaries (observe how long each orbit takes)  Spectroscopic – we see the Doppler shift in the light from the system as the two stars orbit one another (the time it takes the spectral lines to shift back and forth)  Eclipsing – we see the system dim as the stars eclipse one another (the time between eclipses) 6. Given a set of stellar spectra, identify which one corresponds to a specified spectral class.  Table 15.1 (Page 500-501) 7. What information do you need to calculate a star's luminosity? (There are multiple possible answers.)  Apparent brightness + Distance = Luminosity 8. Name two methods for measuring the distances to celestial objects. Describe how each method works.  Using parallax and the relationship between luminosity, distance, and brightness, we can calibrate a series of standard candles (a basis on which to measure apparent brightness of another object – if it is brighter than the standard candle, it is closer to us). We can measure distances greater than 10 billion light years using white dwarf supernovae (SN 1a) as standard candles  Main sequence fitting – We identify a star cluster that is close enough for us to determine its distance by parallax and plot its H-R diagram. We can look at stars in other clusters that are too far away for parallax measurements and measure their apparent brightnesses. If we assume that main-sequence stars in other clusters have the same luminosities as their counter-parts in the nearby cluster, we can calculate their distances from the inverse square law for light. 9. If two star clusters have different main-sequence turn-off points, which one is older?  10.When a star is forming, where does it first appear in the H-R diagram?  Star formation = blue stars (so look at the site of blue stars) 11. If you want to find young stars, what spectral class of stars should you look for?  O (very hot, and young stars) 12.What is the correct order of the stellar spectral classes from coolest to hottest?  (HOT) O B A F G K M (COOL) 13.What are the roles of the disk and jet in the formation of a protostar?  Jet – clears away the cocoon of gas that surrounds the forming star, revealing the protostar within  Disk (accretion disk) – slows down the rotation of the protostar while generating friction and heat, adding to the mass of the protostar (due to shrinking and gas particles dropping onto the surface of the protostar) 14.In large spiral galaxies, which types of main-sequence stars are most common and which are least common?  Disk population – includes stars of all ages and masses that orbit in the disk of the galaxy  Spheroidal population – consists of halo and bulge stars, with the halo stars generally being old and low in mass 15.If two stars have the same mass, but one has lower luminosity, which one will spend more time on the main sequence?  The less luminous star will have a longer lifetime 16.How will the Sun move through the H-R diagram over the course of its main-sequence lifetime?  Protostar – Main sequence – Red Giant – Helium Flash – Double Shell Burning – Planetary Nebula – White Dwarf 17.How do massive stars that are destined to become black holes or neutron stars overcome electron degeneracy pressure?  Within a short period of time (fast) all of the electrons are converted into neutrons … resulting in supernova … but overcoming electron degeneracy pressure 18.What would happen to the Earth if we replaced the Sun with a black hole having exactly the same mass as the Sun?  Nothing 19.What are the stages in the death of the Sun?  Solar core fusion progresses, more and more of the Sun’s core becomes helium  The Sun is not currently hot enough to fuse helium nuclei After 10 billion years on the main sequence, most of the Sun’s core will be “helium ash”  Fusion shuts off and the core stars to cool and contract  Releases energy, formerly stored as gravitational potential energy Hydrogen can only burn in a shell around the core, rate of burning increases  Hydrogen shell burning floods the outer layers of the Sun with more energy than they used to receive, and they expand dramatically  As the Sun expands, the temperature of its surface falls, and the surface changes from yellow to red  The Sun becomes a Red Giant (dead)  When it runs out of hydrogen to burn, the Sun evolves off the main sequence, climbs the sub-giant branch, and becomes a Red Giant 20.What will be the ultimate end state of the Sun?  White dwarf 21.What is at the center of the Milky Way and how do we know?  We cannot see into the centre of the Galaxy in visible light because of the dust, but radio telescopes, x-ray telescopes, and infrared instruments can  Orbits of stars near the centre of our galaxy indicate that it contains a compact object with 4 million times the mass of the Sun  Because the orbits are nice ellipses, that central object must be very small (about 1 AU) -- very massive black hole 22.What's the difference between a neutron star and a pulsar?  Neutron star – what is left over after a supernova (core of star
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