Last Unit

Question 1

What nuclear reactions happen to a star while it is on the main sequence?

Answer 1

Hydrogen fusion to helium

Question 2

What triggers a star to leave the main sequence?

Answer 2

No more hydrogen in the core

Question 3

What happens to the internal events of a star when it leaves the main sequence?

Answer 3

The core collapses, increasing it’s temperature.
The envelope swells, decreasing it’s temperature.
A shell is created where hydrogen is fused.

Question 4

What happens during the helium flash?

Answer 4

The core is now hot enough to fuse helium.

The core becomes larger, shortening the envelope.

Question 5

In a star like our sun what happens when it runs out of helium?

Answer 5

There is not enough mass to fuse carbon, so it will explode into a planetary nebula, ejecting a fraction of the envelope. A white dwarf will remain.

Question 6

What are the properties of a white dwarf?

Answer 6

Very small, very hot star where no more nuclear reactions occur. Little luminosity.

Question 7

What fate awaits stars with less than 5 solar masses?

Answer 7

A white dwarf

Question 8

If a white dwarf gains enough mass and exceeds the Chandrasekhar limit (1.4 solar masses), what will happen to the star?

Answer 8

It will collapse into a neutron star giving rise to a supernova of type Ia.

Question 9

What is the fate of a star with a mass greater than 5 but less than 15 solar masses?

Answer 9

It will become a neutron star.

Question 10

What are the characteristics of a neutron star?

Answer 10

It is very small (~10km) but has a mass between 1.4 and 3 solar masses so it is very dense. It rotates really fast because of the conservation of angular momentum during it’s collapse. It also has a huge magnetic field.

Question 11

How can a neutron star be observed?

Answer 11

As the companion of a star

As a pulsar (similar to a lighthouse)

Question 12

What fate awaits a star should it have a mass greater than 15 times that of our sun?

Answer 12

Collapse into a black hole

Question 13

If the mass of a star is less than 0.08 solar masses, what happens?

Answer 13

Nuclear reactions never start and is therefore called a brown dwarf.

Question 14

What are the building blocks of the universe and how are they organized?

Answer 14

Galaxies, organized in Groups, Clusters and Superclusters.

Question 15

What are the three main types of galaxies?

Answer 15

Spiral galaxies (~60%)
Elliptical galaxies (~35%)
Irregular galaxies (~5%)

Question 16

How do we know we aren’t in the center of the galaxy?

Answer 16

The galactic halo is not centered around the Sun

Question 17

The disk or the halo of a galaxy mostly consists of pop II stars

Answer 17

Halo

Question 18

How many major arms does our galaxy have?

Answer 18

2

Question 19

What is the rotation curve of a galaxy?

Answer 19

He rotation differential (not as a rigid body) the variation of vrot with distance.

Question 20

On what order of years does it take the sun to revolve once around the galaxy?

Answer 20

200 million years

Question 21

What is 90% of the mass of our galaxy?

Answer 21

Dark matter

Question 22

What are some possible candidates for dark matter?

Answer 22

MACHOS (faint stars, failed stars, planets, etc.)

WIMPS (neutrinos? exotic particles?)

Question 23

What is the cosmological principle?

Answer 23

The idea that we are not a special case in the universe.

Question 24

What supports the cosmological principle?

Answer 24

• The homogeneity of matter distribution (same density everywhere)
• The isotropy of the Hubble flow (same in every direction)
• The isotropy of the cosmic microwave background