Astronomy Chapter 21

Astronomers studying regions like the Orion Giant Molecular Cloud have observed that a wave of star formation can move through them over many millions of years. What sustains such a wave of star formation in a giant molecular cloud?

When massive stars form, their ultraviolet radiation and later their final explosions compress the gas in the cloud and cause a new group of stars to form

Astronomers identify the "birth" of a real star (as opposed to the activities of a protostar) with what activity in the star?

When nuclear fusion reactions begin inside its core

The Orion Nebula is

a large cloud of gas and dust illuminated by the light of newly formed stars within it

Astronomers call a ball of matter that is contracting to become a star

a protostar

You are an astronomy graduate student and you are observing the big Orion Nebula from an airplane that has a good-sized infrared telescope built into it (there really is such a plane). On an infrared image of the Nebula, what would particularly stand out?

the clouds of the nebula that have a lot of dust in them

What observations about disks of dusty material around young stars suggest that planets may be forming is such disks?

the disks show lanes that are empty of dust within them

A Herbig-Haro (HH) object is

where a jet from a star in the process of being born collides with (and lights up) a nearby cloud of interstellar matter

Why is it so difficult for astronomers to see new stars in the process of birth?

- most stars are born inside dusty clouds, which block any light that may be coming from the stars - the size of a newly forming star is typically quite small and thus hard to make out - birth happens very quickly, so it is hard to "catch" stars "in the act" - protostars which are not yet doing fusion do not give off a lot of visible light

A graduate student is given the assignment to find stars with duty disks around them. What kind of telescope would it be best for her to use for this purpose

A large telescope that detects infrared radiation

If you want to find stars that are just being born, where are the best places to search?

In giant molecular clouds

Which of the following is NOT one of the reasons planets around other stars are so difficult to detect?

Planets only form very late in the life of a star, just when it is ready to die, and thus last only a very short fraction of the star's life

Which of the following are the small regions that are the embryos of stars (where individual stars are most likely to be born)?

The cores within clumps of molecular clouds

If an astronomer wants to find and identify as many stars as possible in a star cluster that has recently formed near the surface of a giant molecular cloud (such as the Trapezium cluster in the Orion Nebula), what instrument would be the best for her to use?

an infrared telescope (and camera)

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

its mass

Astronomers believe that disks of material will form around protostars that are spinning. Which of the following observed phenomena is a good indication of the presence of a disk around a protostar?

jets and Herbig-Haro objects

In figuring out the evolutionary tracks on the H-R diagram, astronomers

make model stars on a computer and then follow how their characteristics will change with time

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

to the left