astro 5

one per year

a few (2-3) per year

ten per year

20-30 per year

100 per year

70% Hydrogen, 30% Helium.

70% Hydrogen, 28% Helium, 2% heavier elements.

70% Hydrogen, 20% Helium, 10% heavier elements.

50% Hydrogen, 50% Helium

50% Hydrogen, 50% Helium

1%

2%

28%

50%

1-50%, depending on the mass of the molecular cloud

100 molecules per cubic centimeter, 10-30 Kelvin.

300 molecules per cubic centimeter, 10-30 Kelvin.

1000 molecules per cubic centimeter, 10-30 Kelvin.

100 molecules per cubic centimeter, 100-300 Kelvin.

300 molecules per cubic centimeter, 100-300 Kelvin.

H2.

He2

CO

H2O

HHe

1 angstrom.

1 nanometer

1 micrometer

1 millimeter

1 centimeter

Interstellar dust absorbs more red light than blue light, making stars appear redder than their true color.

Interstellar dust absorbs more red light than blue light, making stars appear bluer than their true color.

Interstellar dust absorbs more blue light than red light, making stars appear redder than their true color.

Interstellar dust absorbs more blue light than red light, making stars appear bluer than their true color.

The spectral line shift due to a star's motion through the interstellar medium.

ultraviolet

visible

infrared

x- ray

gamma- ray

It escapes the cloud completely.

It is absorbed by dust grains and heats up the cloud.

It is reflected back onto the protostar, heating it up further.

The blue light is absorbed and the red light transmitted.

It shoots out in bright jets.

density only.

temperature only.

density and temperature.

composition.

gravity

density only.

temperature only.

density and temperature.

composition

thermal pressure

As the cloud becomes denser, gravity becomes stronger and overcomes the pressure buildup.

The pressure is transferred from the center of the cloud to its outer edges where it can dissipate.

Thermal energy is converted to radiative energy via molecular collisions and released as photons.

Excess pressure is released in jets of material from the young stars.

Once the cloud reaches a critical density, the pressure becomes degenerate and independent of temperature.

less massive than the Sun.

more massive than the Sun.

more massive than ten times the Sun

more massive than a hundred times the Sun.

more massive than a thousand times the Sun.

thermal pressure

turbulent motions

thermal pressure

fragmentation

by measuring the amount of interstellar reddening

by measuring the amount of interstellar reddening

by measuring the infrared light emitted by the cloud

by measuring the polarization of starlight passing through the cloud

by measuring the amount by which gravity is reduced

The first generation stars are too faint to be visible now.

The first generation stars formed such a long time ago that the light from them has not yet had time to reach us.

The first generation stars were all very massive and exploded as supernova.

The first generation stars formed with only H and He and therefore have no spectral features.

We do not know how the first generation stars were formed.

Without heavy elements, the clouds could not reach as low a temperature as today and had to be more massive to collapse.

Without heavy elements, the nuclear reactions at the center of the stars would be very different.

Without heavy elements, there was no dust in the clouds and they collapsed faster

The Universe was much denser when the first stars were born.

There were no galaxies when the first stars were born.

10 K

30 k

100 k

300 k

1000 k

when the gravity becomes so strong that photons cannot escape

when excited molecules collide with other molecules before they can release a photon

when the cloud becomes so hot and dense that nuclear fusion begins

when magnetic fields trap the radiation

when the cloud cools down so much that less light escapes than is produced by contraction

The rotation rate remains the same and results in stellar rotation.

The rotation dissipates and any residual is left in small overall rotation of the star.

The rotation rate increases and results in fast rotation of the star.

The rotation rate increases and results in a disk of material around a protostar.

The rotation increases the speed of collapse and produces more massive stars.

protostellar jets

protostellar winds

accretion of material onto the star

relatively slow protostellar rotation

all of the above

when the star is 1 million years old

when the central temperature reaches 1 million Kelvin

when nuclear fusion begins in the core

when the thermal energy becomes trapped in the center

when the stellar winds and jets blow away the surrounding material

1 million years

3 million years

10 million years

30 million years

100 million years

from 1 million years for the most massive stars up to 10 million years for the least massive stars

from 1 million years for the most massive stars up to 100 million years for the least massive stars

from 1 million years for the least massive stars up to 10 million years for the most massive stars

from 1 million years for the least massive stars up to 100 million years for the most massive stars

about 30 million years for all stars, whatever mass

H

H2

H+

H-

dust

when the protostar assembles from a molecular cloud

the instant when hydrogen fusion first begins in the star's core

when the rate of hydrogen fusion within the star's core is high enough to maintain gravitational equilibrium

when a star becomes luminous enough to emit thermal radiation

when hydrogen fusion is occurring throughout a star's interior

Its surface temperature and luminosity increase.

Its surface temperature remains the same and its luminosity decreases.

Its surface temperature and luminosity decrease.

Its surface temperature decreases and its luminosity increases.

Its surface temperature and luminosity remain the same.

Its surface temperature and luminosity increase

Its surface temperature remains the same and its luminosity decreases.

Its surface temperature and luminosity decrease.

Its surface temperature decreases and its luminosity increases.

Its surface temperature and luminosity remain the same

Its surface temperature and luminosity increase.

Its surface temperature remains the same and its luminosity decreases.

Its surface temperature and luminosity decrease

Its surface temperature decreases and its luminosity increases.

Its surface temperature and luminosity remain the same.

when the cloud first begins to contract

when the thermal pressure is trapped at the center of the cloud

when the protostars undergoes convective contraction

when the protostar undergoes radiative contraction

only when the star reaches the main-sequence

2-5

5-10

10-100

100-1000

a million

8 times the mass of Jupiter

80 times the mass of Jupiter

800 times the mass of Jupiter

about 1/80 the mass of our Sun

about 1/800 the mass of our Sun

Brown dwarfs emit infrared radiation.

Brown dwarfs are more massive than Jupiter

Brown dwarfs do not orbit main sequence stars.

The cores of brown dwarfs are supported by degeneracy pressure.

Brown dwarf spectra contain methane absorption lines.

NP

YZ

LT

CD

UV

10 solar masses.

20 solar masses.

50 solar masses.

100 solar masses.

200 solar masses.

molecular clouds do not have enough material to form such massive stars.

they would fragment into binary stars because of their rapid rotation

they would generate so much power that they would blow themselves apart.

they shine exclusively at X-ray wavelengths and become difficult to detect.

they are not bright enough to be seen nearby.

1

3

10

30

100

a cluster of stars that appeared to be 13 billion years old

a 100-solar-mass star

a 0.01-solar-mass star

a molecular cloud without any stars