ASTRONOMY UNIT THREE
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65 Terms
What is apparent magnitude?
The brightness of light from a star or other object as seen from Earth.
A star of apparent magnitude +5 appears how compared to a star of magnitude +3?
Fainter than a star of apparent magnitude +3.
A star of apparent magnitude -1 appears how compared to a star of magnitude +3?
Brighter than a star of apparent magnitude +3.
What is absolute magnitude (M)?
The apparent magnitude a star would have if it were exactly 10 parsecs from Earth.
How does apparent brightness vary with distance?
I ~ 1/dĀ² (inverse square law).
What do spectral types (O
B, A, F, G, K, M) uniquely define? ,Surface temperatures.
What is the Sun's spectral classification?
G2V.
What two physical parameters are plotted on the Hertzsprung-Russell diagram?
Luminosity and spectral type (surface temperature).
What fraction of stars near the Sun are main-sequence stars?
Over 90%.
How can the chemical makeup of the Sun's surface be determined according to your class key?
By taking a sample of the star's surface with a space probe.
Compared to a star in the middle of the HR diagram
a star in the upper right is what?, Larger.
What does the space between stars contain?
Gas, both atomic and molecular, and dust.
Luminosity class (I
II, III, IV, V) is most closely related to what property? ,Radius of the star.
What is a star's luminosity?
The total energy emitted by the star into all space per second, measured in watts.
What shorthand notation denotes stellar luminosity?
Lo.
What is the order of stellar spectral classes from hottest to coldest?
OBAFGKM.
What is the space between stars in our galaxy called?
Interstellar space.
How have complex molecules such as formaldehyde been detected in interstellar clouds?
By molecular emission lines.
Which common interstellar molecule contains nitrogen but no oxygen?
Ammonia (NH3).
What is the most abundant element in the universe?
Hydrogen.
What is the most common molecule in interstellar space?
H2 (molecular hydrogen).
New stars are formed from what?
Huge, cool dust and molecular gas clouds.
What is the typical mass of a giant molecular cloud?
100,000 to 1 million solar masses.
What process occurs inside main-sequence stars?
Hydrogen is converted to helium in their cores.
How is gas distributed in interstellar space?
In clumps concentrated in molecular clouds.
During what phase does hydrogen fuse into helium in a star's core?
The main-sequence phase.
How do massive stars normally end their lives?
They explode.
Where are white dwarfs found on the HR diagram?
Below and to the left of the main sequence.
What property most determines a star's lifetime? Its mass.
At what distance is absolute magnitude defined?
32.6 light-years (10 parsecs).
What is the Crab Nebula?
The remnant of a supernova explosion.
How will the Sun end its life?
It will become a Red Giant and eventually a white dwarf.
Where are stars and planetary systems most likely forming?
In nebulae composed of gas and dust.
How long would it take to reach Proxima Centauri at 50
000 mph?, 57,077 years.
Accretion of matter within an interstellar cloud leads to what?
A protostar.
How far away is the nearest spectral-class V star according to your class key?
9.3 Ć 10ā· miles.
New stars are formed from what?
Huge, cool dust and gas clouds.
Compared to a star in the middle of the HR diagram
a star in the upper right is what?, Larger.
Where are emission nebulae (H II regions) found according to your class key?
In or near old open clusters.
What is a white dwarf?
A small, very hot, low-mass star.
How fast is the speed of light?
186,000 miles per second in every direction.
By what factor does a typical nova brighten?
10ā“ to 10ā¶.
When a main-sequence star was a protostar
it was what?, Larger and cooler.
Enif is spectral class K2 while the Sun is G2. What can be concluded?
Enif is cooler than the Sun.
One of the most recent supernova explosions in our galaxy created what?
Cassiopeia A.
What is the Orion Nebula?
A large interstellar gas and dust cloud containing young stars.
What marks the beginning of the main-sequence phase?
Nuclear reactions begin in the core.
Is Earth a self-luminous object?
No.
What is the lowest mass an object can have and still be a star?
0.08 solar masses.
Why are protostars difficult to observe?
All of the above.
What is the lower edge of the Main Sequence band called?
The zero-age main sequence.
Which location on the HR diagram has luminosity and temperature similar to a T Tauri star?
4.
Which is NOT evidence of the interstellar medium?
Molecular absorption lines in the spectra of cool stars.
Interstellar gas clouds may collapse to form stars if they do what?
Encounter a shock wave.
How can molecular clouds be observed?
Using radio telescopes to observe CO emission.
Due to interstellar dust
a distant star appears how?, Fainter and cooler than it really is.
What is the lowest mass object that can initiate hydrogen fusion?
0.08 solar masses.
As a main-sequence star exhausts hydrogen in its core
its surface becomes what and its luminosity becomes what? ,Cooler; larger.
The Crab Nebula is what?
A supernova remnant.
Why are star clusters important to astronomy?
They let us test theories and models of stellar evolution.
After exhausting hydrogen in its core
a main-sequence star becomes what? ,A red giant star.
What is a planetary nebula?
The expelled outer envelope of a medium-mass star.
What does the Chandrasekhar limit tell us?
White dwarfs more massive than 1.4 solar masses are not stable.
What is a white dwarf composed of?
Carbon and oxygen nuclei and degenerate electrons.
What type of spectrum does a planetary nebula produce?
An emission spectrum.