Astro 150 Spring 2024 Review Trivia (Mar. 1) Lectures 11 and 12

The stars in the bottom left corner of the HR diagram have high temperatures, but they're not very luminous. Why?

• They are large stars

• They've been misplaced; no star really occupies that part of the HR diagram

• They are small stars

• They are brown dwarfs

Which region of the HR diagram has the most stars?

• The giant branch (top right)

• The main sequence (the middle stripe)

• The white dwarf branch (lower left)

Answer explanation

All stars go through a life cycle, and the longest part of any star's life cycle is the main sequence, the part where they are burning hydrogen in their cores to make helium.

If most stars are main sequence stars, why might this diagram seem to have more giants than main sequence stars?

• Only stars that are above a certain brightness are put on this diagram

• Only stars that are above a certain luminosity are put on this diagram

• This diagram is looking at a region of space with mostly giant stars

• Only stars of a certain spectral type are on this diagram

Answer explanation

This problem showcases the issues that come with making brightness limited HR diagrams. Such diagrams tend to suggest there are more giant stars compared to main sequence stars, when in reality there are far more MS stars than giant stars.

What is a key property of stars that determines their evolution and place on the HR diagram?

• Temperature

• Radius

• Luminosity

• Mass

Answer explanation

The mass of stars is the main driver of many of a star's properties, including its placement on the HR diagram.

What's the other key property that determines a star's placement on the HR diagram?

• Composition

• Radius

• Luminosity

• Number of planets

Answer explanation

Over time as a star burns hydrogen into helium, it will move on the HR diagram. Once it has burned enough of its hydrogen, the star will grow and move to the giant branch.

How much more luminous is a 5 solar mass star than a 1 solar mass star?

• 25 times

• 125 times

• 280 times

• 625 times

Why are binary stars so important to astronomers?

• Not much, beyond being interesting physical phenomena

• They help us establish what a spectral type's spectrum really looks like

• Figuring out if there are habitable exoplanets around binary star systems is the next big question in astronomy

• Binary stars help us learn what stars' masses are

Answer explanation

Binary stars are extremely important in astronomy. With some relatively simple calculations, we can find the masses of the components of a binary star system. If we can find the spectral types of stars in a binary system, we can therefore determine the approximate mass of all stars of those spectral types. In other words, binary star systems helped us discover the masses of stars in each spectral type.

Which of these is NOT a type of binary star system?

• Eclipsing binary

• Hubble binary

• Spectroscopic binary

• Visual binary

Eclipsing binaries help us find ________ while spectroscopic binaries help us find _______.

• Temperature, Composition

• Composition, temperature

• Radius, Mass

• Mass, Radius

Answer explanation

Eclipsing binaries help us find the radii of stars. As you see the light vs. time graph of an eclipsing binary system start to drop, the slope of that drop can help you find the eclipsing star's radius.

Spectroscopic binaries help us find stars' masses. As the stars orbit the common center of mass, their spectra swing back and forth due to the Doppler shift. How much those lines swing can tell you the masses of the stars using the see-saw law.

Red giants are more luminous than the sun, but also have the lower temperature. How is this possible?

• Red giants are much larger than the sun

• Red giants are further away from us

• Red giants are much smaller than the sun

• We don't know.

Answer explanation

Luminosity is proportional to R²T⁴ . Having larger luminosity but lower temperature means red giants are much larger than the sun.