Astro150 Trivia - February 23, 2024

What is a parsec?

• A unit of distance that's about 3.26 times a lightyear

• The time it takes for light to travel from here to Alpha Centauri

• 1/12 of the Kessel Run

How does stellar parallax help us find distance?

• The parallax angle is related to the distance of the star

• As we track a star's motion over several years, we can get an idea of its distance

• It doesn't

If star 1 has twice the parallax angle of star 2, which is farther away? By how much?

• Star 1 is farther away. It is twice as far away.

• Star 2 is farther away. It is twice as far away.

• Star 1 is farther away. It is four times farther away

• Star 2 is farther away. It is four times farther away.

• We need more information

What is related to the proper motion of a star?

• The star's overall velocity relative to us (tangential and radial)

• How fast the star is moving away from us (the radial velocity)

• How fast the star is moving across the sky (the tangential velocity)

• The velocity of the star relative to the center of the Milky Way

Why can't we see the proper motions of the stars without specialized equipment?

• We're moving at the same rate as the stars with the spiraling of the Milky Way

• Most stars aren't actually moving at all

• The stars move very slowly

• The stars are extremely far away

How do we know that the Big Dipper looked different in the past?

• We can look at the stars' proper motions and extrapolate them to find where they were in the past

• We have historical record that shows us what the Big Dipper used to look like

• We know that it has always looked the same

• We don't know if it looked different in the past

Answer explanation

We don't expect the proper motions of stars to change very much, so we can use what we know about the stars' current proper motions to estimate where the stars were in the past.

How can we tell that a star is moving toward us or away from us?

• We see the stars getting dimmer or brighter, depending on how they're moving

• We ping radar signals off of the star

• We can see the shift in its spectrum

• We do not have the technology to determine this information

Answer explanation

The radial velocity, or the velocity of a star towards us or away from us, can be determined by shifts we see in the stars' spectrum. This works according to the principles of Doppler shift. As a star moves towards us, its spectral lines shift toward the blue, as a star moves away from us, its spectral lines move toward the red.

If we see a spectral line of a star that is redder than that spectral line is at rest, what can we infer about the star's motion?

• The star is moving away from us

• The star is moving toward us

• Something's wrong; spectral lines don't shift toward the red

What CAN'T the Doppler shift tell us?

• The radial velocity of a star

• How fast a star is rotating

• If a star has planets

• Doppler shift can tell us all these things.

(Review): Star A has luminosity of L and brightness of B at distance d from us. If the distance becomes two time larger (2d), how the luminosity and the brightness change?

• nothing changes

• L, B/4

• L/4, B

• L, B/2