Chapter 15

This statement does not make sense. Blue stars are hotter than red stars.

Stars that look red have hotter surfaces than stars that look blue.

Giant and supergiant stars are stars that have left the main sequence after exhausting their supplies of hydrogen fuel in their central cores. They release fusion energy so furiously that they have to expand in order to radiate it away. When a star that is similar in mass to our Sun runs out of fuel completely, it forms a dead core in which the nuclear fusion has ceased. This core is called a "white dwarf." They are small (about the size of Earth) and very dense. Because they are basically exposed stellar cores, they are also very hot.

how do giants and supergiants differ from main sequence stars? what are white dwarfs?

B. 6 meters (the size of a dorm room)

suppose we wanted to represent all of these objects using the 1-to-10-billion scale from chapter 1, on which the sun is approximately the size of a grapefruit. Approx. how large in diameter would the sun Aldebaran be on this scale?

This statement is true. Giants, supergiants, and white dwarfs are later stages in the evolution of stars that began life as main-sequence stars.

All giants, supergiants, and white dwarfs were once main-sequence stars.

This statement does not make sense. Apparent brightness depends on both luminosity and distance.

Two stars that have the same apparent brightness in the sky must also have the same luminosity.

This statement does not make sense because most stars have very similar proportions of elements. Differences among the appearances of stars arise primarily because of differences in age and mass, not element content.

Two stars that look very different must be made of different kinds of elements

luminosity: about 10Lsun lifetime: approximately 1 billion years

What are the approx. luminosity and lifetime of a star whose mass is twixe that of the sun?

luminosity: about !0,000Lsun lifetime: slightly longer than 10 million years

What are the approximate luminosity and lifetime of a star whose mass is 10 times that of the Sun?

luminosity: about 100Lsun lifetime: slightly shorter than 1 billion years

What are the approximate luminosity and lifetime of a star whose mass is 3 times than that of the Sun?

apparent brightness and distance

What do we need to measure in order to determine a star's luminosity?

the time between eclipses and the average distance between the stars

What two pieces of information would you need in order to measure the masses of stars in an eclipsing binary system

a cluster whose brightest main sequence stars are yellow?

Which of these star clusters is oldest

a cluster of stars containing all colors

Which of these star clusters is youngest

A 30mSun main sequence star

Which of these stars has the greatest surface temperature?

a main sequence M star

Which of these stars has the longest lifetime?

a main sequence A star

Which of these stars is the most massive?

C. 1 millimeter

Approximately how large in diameter would the star Procynon B be on this same scale?

get smaller

If the star Alpha Centauri were moved to a distance 10 times farther than it is now, its parallax angle would

This statement is false. Most of the stars are less massive than the Sun. Many more low-mass stars are formed than are high-mass stars. The high-mass stars burn out sooner, too, while the low-mass stars persist for billions of years.

Most of the stars in the sky are more massive than the Sun.

This statement makes sense. The parallax for Alpha Centauri is larger than that for Sirius because Alpha Centauri is closer to us.

Sirius looks brighter than Alpha Centauri, but we know that Alpha Centauri is closer because its apparent position in the sky shifts by a larger amount as Earth orbits the Sun.

This statement does not make sense. All main-sequence stars are converting hydrogen to helium.

Some of the stars on the main sequence of the H-R diagram are not converting hydrogen into helium.

This statement makes sense. Clusters with no blue stars probably had some blue stars in the past, but as the clusters aged the blue stars rapidly died off.

Star clusters with lots of bright, blue stars of spectral type O and B are generally younger than clusters that don't have any such stars.

This statement is false. The most massive stars burn their fuel a lot faster than the conservative, low-mass stars. They burn fuel at a profligate rate that negates their size/mass advantage.

Stars that begin their lives with the most mass live longer than less massive stars because they have so much more hydrogen fuel.

This statement makes sense. Temperature on the H-R diagram increases from right to left, and stellar radii on the same diagram increase diagonally from lower left to upper right. So the smallest, hottest stars are in the lower left-hand corner of the H-R diagram

The smallest, hottest stars are plotted in the lower left-hand portion of the H-R diagram

D. Three kilometers. (the size of a small town)

approx. how large in diameter would the star Betelgeuse be on the same scale?

Stellar parallax is the tiny movement of stars in our sky due to Earth's motion around the Sun. Since more distant stars show smaller parallaxes than closer ones, we can measure the amount that stars move over 6 months (half of an Earth orbit) and find the distance to the stars. Once we know this, we can use the apparent brightness of the star along with the inverse square law for light to determine the star's luminosity.

how do we use stellar parallax to determine a star's distance, and how can we then determine its luminosity?

Pulsating variable stars vary in brightness because they cannot achieve a steady equilibrium. Their upper layers are too opaque, trapping photons beneath them, so they expand. But the expansion results in upper layers that are too transparent, so the photons escape, and the layers contract again. This cycle of expanding and contracting causes the luminosity to vary.

how does the luminosity of a pulsating variable star change with time?

The spectral sequence was discovered from work done by a large number of people. It began with wealthy astronomer Henry Draper, who was a pioneer in stellar spectroscopy. When he died, his widow donated money to the Harvard College Observatory, where Edward Pickering used the money to hire a group of "computers," mostly women, who studied the stellar spectra. One of Pickering's employees, Williamina Fleming, found that she could classify the spectra by the strength of their hydrogen lines. She ordered them A, B, C, . . . , O. However, this scheme proved inadequate to shedding any understanding on the nature of stars. Annie Jump Cannon realized that she could reorganize the system into a more natural order, in the process eliminating or combining some of the original classes. This left us with OBAFGKM.

how was the spectral sequence discovered, and why does it have the order OBAFGKM? which stars are hottest and coolest in this sequence?

There are three kinds of binary star systems. The first is visual binaries, systems in which we can see both stars distinctly as they orbit each other. The second type of binary system is the eclipsing binary, which we see by examining the light curve. Light curves of eclipsing binaries show periodic dimming, corresponding to when one of the stars passes behind the other and its light is blocked. The final type of binary is the spectroscopic binary. For these systems, we detect the presence of two stars (rather than one) by the Doppler shifts in the spectral lines. Eclipsing binaries are particularly important for finding stellar masses because we can measure the orbital periods of the stars and the velocities. (We can get the velocities in this case because we know that these systems orbit in the plane of our line of sight.) With this information, we can determine the orbital separation and then the masses via Newton's version of Kepler's third law.

what are the three basic types of binary star systems? why are eclipsing binaries so important to measuring masses of stars?

Luminosity classes of stars are designated by Roman numerals and tell us what region of the H-R diagram the star falls in. We use both spectral type and luminosity class to completely classify stars; the spectral type tells us the star's temperature while the luminosity class tells us its radius. So, for example, our Sun is a G2 V, where G2 is the spectral class (indicating that the Sun is a yellow-white star) and V is the luminosity class (telling us that the Sun is a main-sequence star).

what do we mean by a stars luminosity class? explain how we classify stars by spectral type and luminosity class

Spectral types are a way of classifying stars according to their color or what spectral lines we see in their light. The spectral types run OBAFGKM, where O stars are the hottest and M are the coolest. Hotter stars look bluer to us, and cooler stars look redder.

what do we mean by a stars spectral type, and how is spectral type related to surface temperature and color?

The defining characteristic of a main-sequence star is that it falls along a specific line on the H-R diagram and so it exhibits a particular relationship between luminosity and surface temperature. This relationship occurs because the more luminous stars have larger masses and therefore have higher rates of fusion in their cores. Because of the particular relationship between luminosity and radius along the main sequence, more massive stars must also be much hotter than less massive ones in order to emit their energy from their surfaces. (Hotter surfaces emit more light per unit area.)

what is the defining characteristic of a main-sequence star? explain why massive main sequence stars are more luminous and have hotter surfaces than less massive main sequence stars

a k star

which of these stars has the coolest surface temperature

a supergiant M star

which of these stars has the largest radius?

Smaller stars have longer lifetimes than larger stars. This is because the larger stars are much more luminous than the smaller ones. While the larger stars have more fuel to use up, their luminosities are so great that they consume their fuel supply faster and end their main-sequence lives sooner. If stellar luminosities were simply proportional to stars' masses, all stars would have the same lifetimes. But massive stars are much more luminous compared to their mass than are low-mass stars.

which stars have longer lifetimes: massive stars or less massive stars?

A star's birth mass determines its quantity of hydrogen fuel, its central pressure and temperature, and therefore its luminosity. These in turn set the star's main-sequence lifetime and surface temperature. These relationships mean that the birth mass of the star sets most of its other properties.

why is a star's birth mass its most fundamental property?