Principles of Astronomy Quiz 11

Which of these star clusters is oldest?

A cluster whose brightest main sequence stars are yellow.

Which of these star clusters is youngest?

A cluster whose brightest main-sequence stars are white.

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

A star's mass determines surface temp and luminosity throughout a star's main sequence life which lead to the star's lifetime.

Which of these stars is the most massive?

A. a main sequence A star because its closer to O (most massive stars also have hotter cores!)

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

A. the time between eclipses and the average distance between the stars

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

B. get smaller

Which of these stars has the longest lifetime?

C. A main sequence M star (the hotter the star is the shorter its life, cant be O).

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

True, because Sirius is a much bigger star regardless of the distance. It is so much brighter than Alpha Centauri that it appears brighter even from a greater distance.

What do we mean by a star's spectral type, and how is spectral type related to surface temperature and color? Which stars are hottest and coolest in the spectral sequence OBAFGKM?

spectral type- a way to classify the surface temperature from the spectral lines present in a star's spectrum (the letters); the hotter the temperature are bluer in color and cooler in temperature are redder in color; O is the hottest and M is the coolest

How do giants and supergiants differ from main-sequence stars? What are white dwarfs?

- Super/giants have exhausted the hydrogen in their cores and no longer generate energy by fusing hydrogen; rapid rate of fusion----> expansion of outer layers and increased luminosity -white dwarfs: remaining, fusion-less cores of giants; very dense; no energy source; radiate left-over heat into space

Which of these stars has the coolest surface temperature?

C. K because its closer to M

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

C. apparent brightness and distance

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

False. Temperature has a bigger effect on the appearance of the star.

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

True, hottest is close to the O in OBAFGKM, and if its small its low.

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

True, hydrogen has been used up and the stars have moved into late middle age.

What do we mean by a star's luminosity class? Briefly explain how we classify stars by spectral type and luminosity class.

luminosity class describes the region of the H-R diagram in which the star falls; related more to size than luminosity (I = supergiants, III = giants, V = main sequence) -spectral type = surface temp and color -luminosity class = radii and luminosity

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?

-clusters at different ages have different turnoff points -age of the cluster is equal to the lifetimes of stars at the main sequence turnoff point

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

-hydrogen burning cores -mass sets the fusion rate of stars; lower fusion rate = less luminous

Describe in general terms how open clusters and globular clusters differ in their numbers of stars, ages, and locations in the galaxy.

-open clusters are moderately sized, found in the disk of a galaxy, and are young in age -globular clusters are densely packed, found in the halo of a galaxy, and are older.

Which of these stars has the largest radius?

A super giant M star because determined by luminosity.

Which of these stars has the greatest surface temperature?

A supergiant A star

What is a molecular cloud? Briefly describe the process by which a protostar forms from gas in a molecular cloud.

Molecular cloud is a cool, dense interstellar cloud in which the low temperature allow hydrogen to pass up into hydrogen molecules. When a huge molecular cloud starts to collapse, gravity pulls the gas toward the cloud's densest regions, which causes it to break into smaller pieces that each form one or more stars which is a protostar.

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

True, giants make it to the sequence in 2 million years, the sun like ones in 20 mil. Main sequence is the period of like of star where hydrogen convert to helium.

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

False, all of the main-sequence stars on the main-sequence of the HR diagram convert hydrogen into helium.

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

False, because the larger stars have more mass, which pushes down on the core, heating it up, this heating causes an increase in fusion. So more hydrogen is "used" than for a less massive star.

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

False, blue have hotter surfaces.

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

False, stars can look brighter and still be more distant (apparent brightness is how bright the star seems to us vs. luminosity doesn't depend on distance.

How is a star's apparent brightness related to its luminosity? Describe the inverse square law for light.

If you double the distance from a source of light, that lights brightness will decrease by a factor of 4 Example: A star is 10 light years away and has an apparent brightness of 2. If that same star was 20 light years away its apparent brightness would be 8. Remember, the higher the stellar magnitude number means the dimmer that star is.

Which stars have longer lifetimes: massive stars or less massive stars? Explain why.

Less massive star have longer lifetimes because they are burning the hydrogen in their cores at a slower rate than more massive stars.

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

Stellar Parallax= the small annual shifts in a star's apparent position caused by Earth's motion around Earth. We can measure s.p by comparing observations of a nearby star made 6 months apart. The nearby star appears to shift against the background of more distant stars because we are observing it from two opposite points of Earth's orbit. If we know a star's distance from its parallax angle, we can calculate its luminosity with the inverse square law for light.

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

The three types are visual binary, spectroscopic binary, and eclipsing binary. When two bodies orbit each other, we can calculate their masses based on the orbital time and the distance between them. When the binaries are eclipsing each other, that helps a to figure out the distance between them and their masses.

In what ways are all stars similar? In what ways can stars differ?

They are all born with 3/4th hydrogen, 1/4 helium, and no more than 2% heavier elements. They differ in size, age, brightness, and temperature.