Astronomy 152 Exam #2 UTK

For a Sun-like star, at stage 6 the protostar's core becomes hot enough to begin fusing hydrogen into helium. At what temperature does this occur?

10 million K

What is the net result of the triple alpha process?

3 helium atoms become 1 carbon atom and energy.

The Andromeda Galaxy is about 2 million light years away. Which of the methods we've discussed so far could be used to measure this distance?

Cepheid Variable Stars

The comet-hunting astronomer who made a list of over 100 nebulae and galaxies that could be mistaken for comets was

Charles Messier

Astronomers use the term interstellar medium to refer to

Gas and dust that lie between stars

The period-luminosity relationship for Cepheid variables was discovered by

Henrietta Leavitt

Protostars are enveloped by gas and dust as they form, but strong stellar wind escaping the poles of the protostars can collide with and excite gas up to 1 light year away. These objects are known as

Herbig-Haro (HH) objects

Many nebula appear red in photographs because of what element?

Hydrogen

Why are stars unable to fuse iron into larger elements?

Iron is the most stable atomic nucleus, no energy is given off by fusion or fission.

A highly-evolved, Sun-like star fusing helium in its core and fusing hydrogen in a shell around the core is in the horizontal branch of the H-R diagram. What happens when it runs low on helium?

It returns to a red-giant-like state known as a asymptotic branch giant.

Why is it useful for astronomers to study star clusters?

Stars in clusters were born at the same time and are the same distance away, and only differ by mass.

Which is the correct order of stages and events after a sun-like star leaves the main sequence?

Subgiant, red giant, helium flash, horizontal branch giant.

Once almost all available material has been accreted and the central protostar has reached nearly its final mass, it is known as what kind of object?

T Tauri Star

Why does the triple alpha process require much higher temperature and density than the proton-proton chain?

The beryllium-8 isotope is highly unstable and decays quickly.

How is the helium flash different for the most massive stars?

The helium flash doesn't occur in large stars, helium begins fusing gradually.

Why do reflection nebula appear blue?

The nebula scatters blue light more than red light

Heliocentrism was rejected by Aristotle because of the lack of observed stellar parallax. The Copernican Revolution during the 16th and 17th centuries proved heliocentrism to be true, so why did it take until 1838 to measure the first stellar parallax?

The stars are so far away that their annual shift of position in the sky is too small to see without a good telescope.

UV light at the intensities found in the interstellar medium usually keeps molecules from forming. What protects molecules found in the interstellar medium?

They form in clouds that contain dust that UV doesn't penetrate

Astronomers were surprised to detect clouds of gas that were millions of Kelvin in temperature. What is the cause of this high temperature?

Very powerful shock waves from exploding stars heat the gas.

When astronomers use the term nebula, what are they talking about?

a giant cloud of gas and dust between or among the stars

Which instrument would and astronomer use to detect the presence of molecules in a nebula?

a radio telescope

To observe a cool cloud of neutral hydrogen (H I), what instrument would be most useful?

a radio telescope, tuned to a wavelength of about 21 centimeters

As a star forms from a molecular cloud, it

contracts and warms

If an astronomer wants to find the distance to a star that is not variable and is located too far away for trigonometric parallax measurements, he can:

find the star's luminosity class from its spectrum and read the luminosity from an H-R diagram

Within what objects are most stars born?

giant molecular clouds

At stage 7, a Sun-like star is in hydrostatic equilibrium on the main sequence. What two forces are balanced when a star is in hydrostatic equilibrium?

gravity and pressure

For a sun-like star, during the red giant phase the star is running out of hydrogen in its core. The core contracts since less heat is available from fusion, but this causes the core to shrink which heats it. What event ends the red giant phase?

helium flash

When a star settles down to a stable existence as a main-sequence star, what characteristic determines where on the main sequence in an H-R diagram the star will fall?

its mass

Where on the H-R diagram would you find a dim, hot star?

lower left

Where on the H-R diagram would you find a dim, cool star?

lower right

An astronomer is observing a single star (one that does not vary) which she knows is located about 30 light-years away. Which of the following is the most likely method she or her colleagues used to obtain that distance?

measuring the star's parallax

Stars spend how much of their lifetimes fusing hydrogen in their cores?

most

Where can "ZAMS" stars be found on the H-R diagram?

on the left edge of the main sequence

As astronomers use the term, the (trigonometric) parallax of a star is

one-half the angle that a star shifts when seen from opposite sides of Earth's orbit

In the stages of Sun-like star formation discussed in class, at which stage does the protostar appear on the H-R diagram?

stage 4

The higher the luminosity a Cepheid variable is,

the longer the period of its brightness variations

Electron spin-flip in the ground state of a cool, neutral hydrogen atom produces 21.1 cm radiation. When observing this line, it is not always measured at 21.1 cm. Why is this?

the nebula is moving toward or away from us

The apparent brightness of stars in general tells us nothing about their distances; we cannot assume that the dimmer stars are farther away. In order for the apparent brightness of a star to be a good indicator of its distance, all the stars would have to be:

the same luminosity

A star whose temperature is increasing but whose luminosity is roughly constant moves in what direction on the H-R diagram?

to the left

Where on the H-R diagram would you find a bright, hot star?

upper left

Where on the H-R diagram would you find a bright, cool star?

upper right

Astronomers identify the "birth" of a star with what activity in the star?

when nuclear fusion reactions begin inside its core

The luminosity class of a star tells an astronomer

whether the star is a supergiant, a giant, or a main-sequence star

Very late in its life, a sun-like star consists of two parts: a small, extremely dense core known as a _____________ and a hydrogen envelope about the size of our solar system called a ______________ .

white dwarf, planetary nebula