Astronomy Test 2 Review

When we look up in the night sky, some stars look brighter to us than others.  One star may be of the first magnitude while another may be of the fourth magnitude.  If we measure the amount of light reaching us from a third star, we may receive 1.35 x 10^-9 Watts per square meter (1.35 billionths of a W/m²), while we receive 4.92 x 10^-12 W/m² from another star.  These are different ways of describing the stars’ ________________.

• surface temperature

• luminosity

• colors

• mass

• apparent brightness

Harvey reads an article at space.com about a certain star that has 350 L_sun.  This means the star …

• appears 350 times brighter than the Sun when Harvey looks at it in the nighttime sky

• is 350 times more massive than the Sun

• emits 350 Watts of light energy each second

• emits 350 times more light energy per second than the Sun

• is 350 times hotter than the Sun

You measure Star Alif to have three times the apparent brightness of Star Ba. If they are at the same distance from you, what can you say about their luminosities?

• Star Alif has one ninth the luminosity of Star Ba.

• Star Alif has three times the luminosity of Star Ba.

• Star Alif has nine times the luminosity of Star Ba.

• Star Alif has one third the luminosity of Star Ba.

• Star Alif has the same luminosity of Star Ba.

Star Gim has the same apparent brightness as Star Ha, but Gim is 10 times farther away from you than Ha. What can you say about their luminosities?

• Star Gim has 100 times the luminosity of Star Ha.

• Star Gim has the same luminosity of Star Ha.

• Star Gim has one hundredth (1/100) the luminosity of Star Ha.

• Star Gim has 10 times the luminosity of Star Ha.

• Star Gim has one tenth (1/10) the luminosity of Star Ha.

Barney takes color photos of four stars: one is one is pale orange, one is pale blue, one is whitish, and one is orangey-red.  Place these stars in order from coolest to hottest?  [Notice that the bold text may change in your Canvas question].

answer from coolest - hottest

• orangey-red, whitish, pale orange, pale blue

• whitish , pale blue, pale orange, orangey-red

• pale blue, orangey-red, whitish, pale orange

• pale orange, pale blue, orangey-red, whitish

Harvey reads an article at skyandtelescope.com about a certain star that has a K spectral class.  This means the star’s surface temperature is …  [Remember that text values in bold may differ in the question Canvas asks you … be alert and adjust your answer accordingly.]

• considerably cooler than the Sun, around 2500 K

• somewhat hotter than the sun, around 9000 K

• about the same temperature as the Sun, around 6000 K

• considerably hotter than the Sun, around 20,000 K

• somewhat cooler than the Sun, around 4500 K

Our OpenStax textbook talks about the numbers of stars with different spectral types that have been discovered in the “local neighborhood.”  How many stars (including white dwarfs and brown dwarfs) have been discovered within 21 ly of the Sun, and what percentage of them have G spectral classes like the Sun?

• 5 stars found, 40% are G-type

• 75,216 stars founds, about 35% are G-type

• over 100 billion stars (too many to count), roughly 15% are G-type

• 162 stars found, about 4% are G-type

There are no luminous stars with O or B spectral types within 21 ly of the Sun because …

• they do not occur in visual binaries, so we cannot determine their masses

• their apparent brightness are too faint to see

• their angular diameters are too small to see

• they are rare; you might need to sample thousands or millions of stars before you would expect to find one

Our textbook asserts that brown dwarfs are very difficult to find, even if they are quite close to us. The reason for this is that brown dwarfs

• have very high surface temperatures

• have very low luminosities

• release energy in their cores by fusing iron atoms rather than hydrogen atoms

• have very low masses

It is relatively easy to determine the linear diameter of the Sun (you have done it!), but it is difficult to measure the diameters of other stars using the same method because …

• Earth’s atmosphere blocks out the wavelengths of light that are critical for making this measurement

• stars are too close for parallax to work effectively

• Kepler’s Laws do not apply to stars, only to planets in our Solar System

• Newton’s Law of Gravity uses a different proportionality constant G for every star, so it gets too confusing

• other stars are so far away from us that their angular sizes are too small to measure