"What basic composition are all stars born with? Given that all stars begin their lives with this same basic composition why do stars differ from one another?"
All stars are born with the same basic chemical composition of about 75% hydrogen, about 25% helium, and no more than 2% heavier elements. Stars differ from one another because they don't all have the same mass and because we see them at different stages in their lives.
"What do we mean by a star's luminosity? What are the standard units for luminosity? Explain what we mean when measure luminosity in units of LSun."
A star's luminosity is the total amount of power it radiates into space. The luminosity is stated in watts. Our Sun's luminosity is 3.8 x 1026 and so this amount is called LSun. We measure other stars in "solar luminosity" units because it's more meaningful to compare other stars to our Sun.
"Briefly explain how we calculate a star's distance in parsecs by measuring its parallax angle in arcseconds."
The parallax angle is half of the star's apparent shift back and forth over a period of time. A parallax angle of one arcsecond equals one parsec. The distance in parsecs is calculated as 1 divided by the parallax angle (as stated in arcseconds). For example, a star with a parallax angle of 1/5 arcsecond is 5 parsecs.
"Briefly describe how a star's spectral type is related to its surface temperature. List the seven basic spectral types in order of decreasing temperature. How are the spectral types subdivided by number? (e.g. A3, G5)?"
The seven spectral types are O, B, A, F, G, K, M, in order from hottest (over 30,000 K) to coolest (less than 3500 K). They are in this order because at first the stars were classified according to the strength of their hydrogen lines. Later it was discovered that a better system came from using just OBAFGKM classifications in that order, and eventually it was understood that the reason the stars followed this pattern wasn't because of different compositions but because OBAFGKM relates to surface temperature, and the reason the O types had such weak hydrogen lines was because their immense heat had led to increased ionization.
Each of the spectral types are further subdivided from 0 to 9, 0 being the hottest and 9 being the coolest. Therefore a G0 star is hotter than a G5 star, but cooler than a F9 star.
"Describe and distinguish between visual binary systems, eclipsing binary systems, and spectroscopic binary systems."
Visual binary systems are pairs of stars orbiting each other that can be seen distinctly without a telescope.
Eclipsing binary systems are orbiting pairs that appear to be a single star rising and fading in brightness. The "star" appears dim when one star eclipses the other. This patterns of fluctuating brightness can be graphed on a light curve in order to detect the presence of the eclipsing system.
Spectroscopic binary systems are orbiting pairs that are detected by Doppler shifts in their spectral lines. If we see two sets of lines displaying blueshifts and redshifts as the two stars move toward and away from us, this is a double-lined spectroscopic binary. If there is only one set of shifting lines because one of the stars is too dim to be detected, this is a single-line spectroscopic binary.
"Describe how we measure stellar masses and why eclipsing binaries are so important to such measurements."
Stellar masses are measured by determining the star's orbital period and its semimajor axis. Using Newton's version of Kepler's third law, the mass of two binary system stars combined will be equal to (4pi2 / G) * (a3 / p2). (a = semimajor axis, p = orbital period, G = gravitational constant) Then by looking at how much the spectral lines shift for each star compared to one another, the mass of each individual star is calculated.
Eclipsing binaries are important to measuring stellar masses because we can determine their true orbital velocities from Doppler shifts and thus accurately measure the separation of two stars. The data is more accurate than the Doppler shifts of other stars because these stars are in our line of sight and we can see how fast they move when they eclipse each other.
Week 04
"Draw a sketch of a basic Hertzsprung-Russell diagram (H-R diagram). Label the main sequence, giants, super-giants, and white dwarfs. Where on this diagram do we find stars that are cool and dim? Cool and luminous? Hot and dim? Hot and bright?"
On the H-R diagram, cool and dim stars are in the lower right area. Cool and luminous stars, such as the red supergiants, are in the upper right. Stars that are hot and dim are in the lower left area. The hot and bright stars are in the upper left area. (See attached for H-R sketch.)
"Explain how mass determines the luminosity and surface temperature of a main-sequence star. How do masses differ as we look along the main sequence on an H-R diagram?"
Along the main sequence of the H-R diagram the blue stars in the upper left area have more mass than the red stars in lower right. The greater the mass of the star, the greater its surface temperature and the greater its luminosity. Mass is the important factor in luminosity because the more the outer layers weigh, the more nuclear burning takes place in a star's core to maintain gravitational equilibrium.
"What do we mean by a star's main sequence lifetime? Explain why more massive stars lead shorter lives."
A star's main sequence lifetime is the time when it still has hydrogen in its core to fuse into helium. The more massive stars have shorter main sequence lifetimes because their great mass and luminosity leads to faster hydrogen consumption.
"What are the Cepheid variables? What is the period-luminosity relation for Cepheids, and why is it useful for distance measurements?"
The Cepheid variables are a special category of very luminous pulsating variable stars. A variable star is one that varies significantly in brightness with time. Some variable stars struggle with maintaining gravitational equilibrium, and because of their ongoing rise and fall in luminosity are called pulsating variable stars.
Most of these stars lie in an area called the "instability strip" on the H-R diagram, between the main sequence stars and the red giants. The most luminous are at the top and are called the Cepheids after the first discovered star of this type (Delta Cephei).
The luminosity changes for Cepheids are between a few days to a few months. The most luminous stars have the longest periods, which is what is meant by the period-luminosity relation. Once the period of the star is measured, the luminosity can be determined. Then the distance can be calculated using the luminosity-distance formula. (Brightness = luminosity / 4pi(d2) Cepheids are useful for measuring distances to other galaxies.
"Describe in general terms how open clusters and globular clusters differ in their numbers of stars, ages, and locations in the galaxy. Why are star clusters so important to astronomers?"
Open clusters usually contain several thousand stars and are always found on the galaxy's disk. Globular clusters can contain over a million stars and are found in both the halo and the disk. Open clusters are typically 30 light-years across whereas globular clusters can span 60-150 light-years.
Star clusters are useful to astronomers because all the stars in one cluster lie about the same distance from the Earth and all formed at (relatively) the same time. This makes them valuable for comparing properties of stars and measuring distances and ages.
"Explain why H-R diagrams look different for star clusters of different ages. How does the location of the main sequence turnoff point tell us the age of the star cluster?"
Older star clusters, such as the Pleiades example on p488 in our text, have a different H-R diagram because the main sequence O stars may have already consumed their hydrogen and thus won't be present on the chart. Likewise, younger star clusters may not have main sequence M stars on the diagram because none of its stars have cooled that much yet.
The main sequence turnoff point is where the star cluster's main sequence on its H-R diagram diverges from the norm. Wherever the turnoff point occurs, this is the age of the cluster. In the Pleiades example the turnoff is at B6, so the cluster is about 100 million years old.
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Posted by: Brittany | 03 April 2007 at 03:33 PM