chapter 12
Explain why H-R diagrams look different for star clusters of different ages. How does the location of the main-sequence turnoff point tells 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
Describe the general terms how open clusters and globular clusters differ in their numbers of stars, ages, and locations in a 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 longest lifetime?...
A main sequence M star (the hotter the star is the shorter its life, cant be O)
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 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 ???
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. T or F why
True..hydrogen has been used up and the stars have moved into late middle age.
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. T or F why
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.
All giants, supergiants, and white dwarfs were once main-sequence stars. T or F why
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.
The smallest, hottest stars are plotted in the lower left portion of the H-R diagram. T or F why
True, hottest is close to the O in obafgkm, and if its small ins low.
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: hot because they are exposed stellar cores, dim because they lack an energy source, radiate because only their leftover heat into space. they are no larger than Earth but have apprx same mass as out sun
What is the defining characteristics of a main-sequence star? Briefly explain why massive main-sequence stars are more luminous.
-Hydrogen burning cores -MASS sets the fusion rate of stars lower fusion rate=less luminous. ---- mass determines the balancing point at which the energy released by hydrogen fusion in the core equals the energy lost from the star's surface. -A very luminous star must be large or have very high surface temperature. (low mass stars are more common than high mass stars)
Which of these 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
What are the three basic types of binary star systems? Why are eclipsing binaries so important for measuring masses of stars?
Binary Star system: systems in which 2 stars continually orbit each other. 1) visual binary: pair of stars that we can see distinctly as the stars orbit each other. 2) spectroscopic binary: identified through observations of Doppler shifts in its spectral lines (alternating blue and red shifts.) 3) Eclipsing binary pair of stars that orbit in the plane of our line of sight. -When the binaries are eclipsing each other, that helps a to figure out the distance between them and their masses.
Some of the stars on the main-sequence of the H-R diagram are not converting hydrogen into helium. T or F why
False, all of the main-sequence stars on the main-sequence of the HR diagram convert hydrogen into helium.
Two stars that look very different must be made of differnt kinds of elements. T or F and why
False, because temperature has a bigger effect on the appearance of the star.
Stars that begin their lives with the most mass live longer than less massive stars because they have so much more hydrogen fuel. T or F why
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. T or F why
False, blue have hotter surfaces.
Two stars that have the same apparent brightness in the sky must also have the same luminosity. T or F why
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.
Which stars have longer lifetimes: massive stars or less massive stars? Explain why.
Less massive stars have longer lifetimes because they are burning the hydrogen in their cores at a slower rate than more massive stars.
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 HR diagram in which the star falls; luminosity class is more closely related to its size than to its luminosity (I=supergiants. II=Bright giants, III=Giants, IV=Subgiants, V=Main sequence stars). Spectral Type=surface temperature and color Luminosity Class= radii and luminosity
What do we mean by a star's spectral type, and how is it related to the star's surface temperature and color? Which types of stars are hottest and coolest in the spectral sequence OBAFGKM.
Spectral lines can show if there are a lot of ionized elements in a star, which would indicate great heat. Stars with lots of molecules are relatively cool, otherwise the atoms wouldn't survive. O= hottest and M= coolest.
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.
In what ways are all stars similar? In what ways can stars differ?
They all form in great clouds of gas and dust, they begin life with same chemical composition as the Sun: about 3/4 hydrogen and 1/4 helium and no more than 2% consisting of elements heavier than helium. They DIFFER in size, age brightness, and temperature.
Which of these stars is the most massive?....
a. a main sequence A star because its closer to O (most massive is also 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 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
Draw a sketch of a basic Hertzsprung-Russel' (H-R) diagram. !!! sketch! #7
on paper
Briefly explain how we can learn about the lives of stars even though their lives are far longer than human lives.
we can examine clusters of stars and compare younger and older stars to see how they look at various stages of life. We can use these observations to form models.
How is a star's apparent brightness related to its luminosity? Explain by describing the inverse square law for light.
we can understand the difference between apparent brightness and luminosity by thinking about a 100-watt light bulb. the bulb's apparent brightness depends on your distance from it. Even thought the light bulb always puts the same amount of light-its luminosity doesn't vary. Inverse Square Law: apparent brightness= luminosity/4pie X distance(squared) *****