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The amount of energy radiated by a star per second.

Star Brightness

Brightness of a star depends on both distance and luminosity.


Another term for brightness of stars.


The apparent change in position of an object when seen from different places.


A group of stars held together by gravity.

Example of spiral galaxy

Milky Way

Example of starburst galaxy



Galaxies grouped together rather than spread apart.

What Cluster does the Milky Way belong to?

The Local Group (5 million light years across)

Super Clusters

Clusters of galaxies that come together (can be 100 million light years across)

Why can stars emit light?

At the center of most stars hydrogen is being converted into helium through a pressure called nuclear fusion, this creates a huge amount of energy.

How hot is the core of the sun said to be?

13,600,000 degrees kelvin

How hot is the Photosphere said to be?

6,000 degrees kelvin

Rank in order of most abundant in the Sun.

Hydrogen, Helium, Oxygen, Carbon, Iron


The unit astronomers use to measure distances in space.

Spectral Type


What is the equatorial diameter of the Earth?

About 10,000 km

What is the equatorial diameter of the Moon?

About 3,000 km

What is the equatorial diameter of the Sun?

About 1,000,000 km

Approximately how many times larger is the size of the Sun compared to the size of the Earth?

About 100x

When does a newly forming star have the greatest luminosity?

When it is a shrinking protostar with no internal fusion

When a newly forming star is at its greatest luminosity, what is its energy source?

Gravitational Contraction

Which of colors of light passes most easily through interstellar clouds?

Longer wavelengths. It passes through interstellar clouds much easier than higher wavelengths visible light.

Protostar Phase

pressure and gravity and not precisely balanced
radius much lager than the Sun
energy generated by gravitational contraction
luminosity much greater than the Sun

Main-sequence phase

surface radiates energy at same rate that core generates energy
lasts about 10 billion years
energy generated by nuclear fusion

How can we see through the interstellar medium?

By observing in high-energy wavelengths such as X rays and long wavelengths of light such as radio waves

What do we mean by the interstellar medium?

The gas and dust that lies in between the stars in the Milky Way galaxy

The interstellar clouds called molecular clouds are _______.

the cool clouds in which stars form

Which of the following types of molecule is the most abundant in an interstellar molecular cloud?


Interstellar dust consists mostly of _____.

microscopic particles of carbon and silicon
These tiny, solid particles are very important, even though their total mass makes up less than 1% of the interstellar medium.

Which part of the electromagnetic spectrum generally gives us our best views of stars forming in dusty clouds?


Most interstellar clouds remain stable in size because the force of gravity is opposed by _______ within the cloud.

thermal pressure
Thermal pressure is the pressure resulting from the thermal motions of the particles in the cloud.

What kind of gas cloud is most likely to give birth to stars?

a cold, dense gas cloud
This type of cloud has lower thermal pressure (due to the low temperature) and stronger gravity (due to the high density), giving gravity the upper hand.

What law explains why a collapsing cloud usually forms a protostellar disk around a protostar?

conservation of angular momentum

What can we learn about a star from a life track on an H-R diagram?

the surface temperature and luminosity the star will have at each stage of its life

A star's particular life track depends only on its mass.

Approximately what core temperature is required before hydrogen fusion can begin in a star?

10 million K

Which star spends the longest time in the protostellar phase of life?

Lower mass stars take longer in all phases of life.

What is the approximate range of masses that newborn main sequence stars can have?

0.1 to 150 solar masses

When does a protostar become a main-sequence star?

when the rate of hydrogen fusion becomes high enough to balance the rate at which the star radiates energy into space

What effect are magnetic fields thought to have on star formation in molecular clouds?

They can help resist gravity, so that more total mass is needed before the cloud can collapse to form stars.
The magnetic fields that thread clouds inhibit the movement of gas, thereby adding to the resistance to gravity provided by thermal pressure (and turbulent gas motions).

Which of the following statements is probably true about the very first stars in the universe?

They were made only from hydrogen and helium.

The vast majority of stars in a newly formed star cluster are ______.

less massive than the Sun

Which of the following statements about brown dwarfs is not true?
(a)Brown dwarfs eventually collapse to become white dwarfs.
(b)Brown dwarfs form like ordinary stars but are too small to sustain nuclear fusion in their cores.
(c)Brown dwarfs are supported against gravity by degeneracy pressure, which does not depend on the object's temperature.
(d)All brown dwarfs have masses less than about 8% that of our Sun.

(a)Brown dwarfs eventually collapse to become white dwarfs.
The terms "brown dwarf" and "white dwarf" may sound like they should be related, but they are not. Brown dwarfs will never collapse into white dwarfs.

What happens within a contracting cloud in which gravity is stronger than pressure if its temperature remains the same as it contracts?

It breaks into smaller fragments.

What is the minimum mass for a star, and why can't objects with lower masses be true stars?

An object must reach a large enough mass that pressure and heat inside its core is high enough to start the process of nuclear fusion. Only then is it considered a true star.

Interstellar medium

Gases (mainly hydrogen and helium) and dust grains (silicates, carbon and iron) filling the space in between stars. The density of interstellar mass is very low. There is about one atom of gas in every cubic centimeter of space. The density of dust is a trillion times smaller. The temperature of the gas is about 100 K.

Molecular Clouds

A cold, dense interstellar cloud which contains a high fraction of molecules. It is widely believed that the relatively high density of dust particles in these clouds plays an important role in the formation and preservation of the molecules

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