Astronomy Mid Term
Using charts found in your textbook, what is the approximate B-V color index of a star with a surface temperature of 5000 K and a luminosity class of 100 Lsun?
+0.81
From the answers below, select two that are current, scientifically-valid methods of determining stellar diameters.
-The time for one star to pass in front of another in an eclipsing binary system is dependent upon the relative diameters of each star. From the light curve, we can measure the time for each star to eclipse the other. We can measure the speed of the stars from the Doppler shift in the spectrum. From knowing the time of eclipse and the speed, the size of each star can be determined. -The time for an object like the Moon to pass in front of a star can be measured to determine the diameter of a star. Since we know the speed of the Moon in its orbit, we can calculate the size of the star.
Which two of the following determine the strength of spectral lines in stellar spectra?
-the star's surface temperature -the star's chemical composition
Which would have a larger diameter: a white dwarf of 1.0 solar-mass or a white dwarf of 0.5 solar-mass?
0.5 solar-mass white dwarf
Which of the following main sequence stars would be the most massive?
09
The suns luminosity is...
1 Lsun
Approximately how long will a G2 star be on the main sequence?
10 billion years
If a cluster shows a main sequence turnoff point at G2, what is the approximate age of the cluster?
10 billion years old
What core temperature is required for helium fusion to occur?
100 million K
If a star has a parallax of 0.01 seconds of arc, what is the stars distance?
100 pc
Astronomers estimate that the ISM makes up about ____ of the total mass of the Milky Way Galaxy.
15%
If a star's degenerate core is greater than about ____ then nothing can stop the core from collapsing forever.
3 solar-masses
The overall fusion reaction by which the Sun currently produces energy is
4 H > 1 He + energy
What is the Sun made of (approximately by mass)?
70 % hydrogen, 28% helium, 2% other elements
Which of the following correctly uses the term "light year"?
A light year is the distance light travels in one year.
What is a CME?
A potentially dangerous burst of charged particles from the Sun that can disrupt satellite communications.
What is the approximate luminosity of a G-type main sequence star?
About 1 Lsun
What causes the radio pulses of a pulsar?
As the star spins, beams of radio radiation sweep through space. If one of the beams crosses Earth, we observe a pulse.
What are the prominent spectral features that identify each stellar spectral class?Be able to identify a star given its most prominent spectral features.
Astronomers have devised a classification scheme which describes the absorption lines of a spectrum. They have seven categories (OBAFGKM) each of which is subdivided into 10 subclasses. ... Although based on the absorption lines, spectral type tells you about the surface temperature of the star.
What proof do we have of ultra-hot ISM temperatures?
Astronomers have recorded X-ray emissions from ISM gas and observed the spectra of oxygen atoms with five levels of ionization.
A star must be hotter than about 25,000 K to produce an H II region. Which of the following stars can produce such regions?
B1V white dwarf O9V
You determine a star has a surface temperature of 22,343 K. What would be the color of the star?
Blue-White
How do we know what goes on under the surface of the Sun?
By both creating mathematical models and measuring Doppler shifts.
Which of the following are true about disks around protostars?
By the time a star is about 10 million years old, the densest parts of most disks have been blown away by the central star. A protostar that is more than about 3 million years old will have a disk of gas and dust, but the dust will reside in the outer part of the disk. The inner region is clear of most dust.
Who showed that the Sun and stars are composed of mostly hydrogen?
Cecilia Payne-Gaposchkin
The first star system that was a candidate for a black hole was .....
Cygnus X-1
Which of the following is closest in size (radius) to a white dwarf?
Earth
True or False? Most of the ISM is composed of ionized hydrogen (HII).
False
True/False: A star that is massive enough to sustain nuclear fusion in its core is a brown dwarf.
False
Which of the following lists the object in order of apparent brightness form brightest to faintest?
Full moon, Sirius, Polaris
Two stars are at the same distance. The stars have spectral classes G2V and G2III. Which star appears brighter?
G2III
Which of the following main sequence stars is most likely to undergo a helium flash later in its evolution?
G9
______ explains gravitation as the interaction of matter in spacetime.
General relativity
Which of the following would be most likely to show a main sequence turnoff point around F9?
Globular Cluster
What was gravitational contraction ruled out as the source of the Sun's energy?
Gravitational contraction would have produced energy for about 100 million years. The Earth is much older than that.
HI refers to atomic hydrogen. HII refers to ionized hydrogen. H2 refers to molecular hydrogen. Which is the most abundant type of hydrogen observed in the universe?
HI
Radio waves at a wavelength of 21-cm are used to detect....
HI regions
Why do HII regions appear red?
HII regions are composed of completely ionized hydrogen. When the hydrogen captures an electron, light is emitted. The red Balmer hydrogen line is the strongest wavelength emitted.
What is the ISM?
ISM refers to the material between stars. Some interstellar material is concentrated into giant clouds known as nebulae.
How do elements such as oxygen and sulfur form?
In the interior of high-mass stars. Through the process of stellar nucleosynthesis.
What makes the Kepler mission so important to exoplanet research?
It is a space-based observatory capable of detecting thousands of exoplanets, many with masses much smaller than Earth.
The first step in the proton-proton chain produces an anti electron, or positron. What happens to the positron?
It is rapidly converted to energy when it meets an ordinary electron, resulting in matter-antimatter annihilation.
Suppose the Sun were to be suddenly replaced by a black hole of one solar-mass. What would happen?
It would get cold and dark, but probably little else.
Who discovered the first pulsars?
Jocelyn Bell
"Matter tells spacetime how to curve; spacetime tells matter how to move," is a quote by whom?
John Wheeler
What is the spectral and luminosity class of a star with a surface temperature of 5000 K and a luminosity class of 100 Lsun?
K0III
Which of the following stars will spend the longest on the main sequence?
K3
Suppose a single star has spectral features indicating it has a high surface temperature that emits mostly blue light, but when observed, the star appears red. What is an explanation for this?
Light from the hot star passes through clouds containing interstellar dust. The dust scatters blue light but allows red light to pass through. The effect is that the star appears red even though it is emitting mostly blue light.
When referring to stars, what is the difference between apparent brightness and luminosity?
Luminosity is the total amount of energy at all wavelengths that the star emits per second. Apparent brightness is the amount of energy that the star emits per second that reaches a given area here on Earth.
Which of the following stars is least luminous?
M, main sequence
When astronomers use the term "metals", they are referring to all elements heavier than hydrogen and helium. The fraction of a star's mass that is composed of these elements is referred to as the star's ___________.
Metallicity
_____ stars are held up by neutron degeneracy.
Neutron
Elements up to iron are fused inside of high mass stars, but how are elements beyond iron created?
Nuclear reactions in the brief period of a supernova explosion can form elements beyond iron.
Which of the following stars is hottest?
O, main sequence
Which of the following main sequence stars is most likely to end in a type II supernova?
O8V
Which of the following correctly orders the spectral types from hottest to coolest?
OBAFGKMY
Which of the following (more than one) are considered supporting evidence of the existence of black holes, even though they are not directly observable?
Observations of x-ray emissions from accretion disks of binary star systems in which a high mass but unseen companion exists. General relativity, the theory upon which black hole theory is built, has been shown to be highly reliable.
The analysis of a diversity of exoplanets has prompted astronomers to update their planetary formation model to include planetary migration. Which of the following are true concerning planetary migration?
Planetary migration is the aspect of planetary formation where a planet's orbit may change to allow it to move closer or farther from its star. Planetary migration is the best explanation of how systems with hot Jupiters may have formed. Planetary migration proposes that Uranus and Neptune formed near the current locations of Jupiter and Saturn, but they moved out to their present locations through gravitational interactions with other planets.
Which of the following stars use the period-luminosity relation to determine distances to galaxies? (More than one possible answer.)
RR Lyrae variables Cepheid variables
An astronomer finds a star with surface temperature of 3000 K with a high luminosity. What kind of star is this?
Since M stars are cool and emit very little energy per unit area (say per square meter), the only way that an M star can have a high luminosity is if it is very large (i.e., has a lot of square meters of surface area). This star is either a giant or a supergiant.
Which statement best describes the "missing" neutrino problem?
Solar neutrinos have been detected, but in fewer numbers than predicted by theoretical models.
Star 1 is a Cepheid variable star whose light curve shows a period of 5 days. Star 2 is an RR Lyrae star whose light curve shows a period of one-half day. If both stars are at the same distance, which will appear brighter?
Star 1
Star 1 is a Cepheid variable star whose light curve shows a period of 5 days. Star 2 is a Cepheid variable star whose light curve shows a period of 25 days. If both stars are at the same distance, which will appear brighter?
Star 2
What would be the source of energy producing hot interstellar regions with temperatures of millions of degrees?
Supernova explosions
Which of the following are likely to occur as part of the evolutionary track of a star like our Sun?
T Tauri stars Herbig-Haro objects protoplanetary disks
Which of the following (more than one) experiments support the predictions of Einstein's general relativity?
The 1919 observation of star displacement during total solar eclipse. The 1959 observation that an atomic clock near Earth's surface runs slower than a twin clock at greater height. The observation that GPS devices, which operate on the assumption that general relativity is true, produce highly accurate positions on Earth's surface.
How does the mass of the Sun compare with the other stars in our stellar neighborhood.
The Sun is more massive than the majority of stars in our neighborhood.
If a fictional newly formed star called "Bravo" emits 28 times more energy than the Sun, yet the Sun appears brighter in the sky, what would be a reason for this?
The Sun is much closer to Earth than Bravo and therefore the Sun's apparent brightness is greater than Bravo.
What is the sunspot cycle?
The average 11 year cycle in which the number of sunspots increases and decreases
What process is required to change an object from a protostar into a zero age main sequence star?
The core temperature of the protostar reaches 10 million K and fusion of hydrogen into helium begins in the core.
What is the best possible solution to the solar neutrino problem?
The electron neutrinos created in the Sun change into another type of neutrino that we do not detect.
What does it mean to say the Suns is in a state of hydrostatic equilibrium
The gravitational contraction of the Sun is balanced by the nuclear pressure due to fusion in its core.
What are the layers of the Sun? How are the layers organized? What are the relative temperatures of the layers? Do these layers apply to other stars?
The inner layers are the Core, Radiative Zone and Convection Zone. The outer layers are the Photosphere, the Chromosphere, the Transition Region and the Corona. IRIS will focus its investigation on the Chromosphere and Transition Region.
Why do older stars have lower heavy-element abundances?
The interstellar medium from which they formed was not enriched with heavy elements formed in stellar interiors.
Two stars are in a visual binary star system. We see the system face on where we can observe the orbits to be perfectly circular. One star is very massive whereas the other is much less massive. Describe their relative orbits in terms of orbit size, period, and orbital velocity.
The larger-mass star would move in a smaller circle around the center of mass since it would be located closer to the center of mass, whereas the low-mass star would be found to have a large orbit farther from the center of mass. Both orbits would be concentric, located around the center of mass. The orbital periods would be the same for each object, similar to the objects on the seesaw—the distance from the center of mass doesn't influence the period. The velocity for the smaller-mass star would be faster than that of the more massive star since it has a larger orbit size compared to the massive star.
Which of the following (more than one) could potentially produce gravitational waves strong enough to detect here on Earth?
The merger of two black holes. The implosion of a very high mass star to form a black hole The merger of two neutron stars.
What determines if an object is a brown dwarf or a star?
The object's internal temperature and whether it can sustain hydrogen fusion in the core
What arethe stellar spectral classes? What temperatures corresponds to each class?What determined these? When? Why?
The spectral types and sub-classes represent a temperature sequence, from hotter (O stars) to cooler (M stars), and from hotter (subclass 0) to cooler (subclass 9). The temperature defines the star's "color" and surface brightness.
If the maximum mass of a white dwarf star is 1.4 solar-masses, then how is it possible for a 6 solar-mass star to end up as a white dwarf.
The star loses mass through strong stellar winds and planetary nebulae.
When an astronomer says a star has a certain temperature, what does that mean?
The temperature refers to the star's surface temperature. The light we receive from a star is emitted from the star's surface. Therefore, the temperature of the surface determines the properties we observe in the light.
Suppose a binary star system consists of a white dwarf star that is gaining mass because of accretion from it companion star. When the white dwarf reaches a mass of 1.4-solar-mass what happens?
The white dwarf undergoes a catastrophic collapse, leading to a type of supernova that is somewhat different from that which occurs in a single massive star but is comparable in energy.
What does the statement "black holes have no hair" mean?
There is nothing about the black hole that tells you what's going on inside.
Which of the following is true concerning HII regions?
They are always found near hot stars which cause the surrounding gas to fluoresce.
Elements heavier than hydrogen and helium are found in the ISM. How are they produced?
They are produced inside stars and then enrich the ISM when the stars die.
Astronomers are able to observe hydrogen in HII regions with optical telescopes. How do astronomers observe cold hydrogen clouds (HI regions) that aren't near a star?
They use radio telescopes and observe the hydrogen through 21-cm radiation.
Two stars are on the main sequence. Star 1 is spectral type B2, while Star 2 is spectral type K0. Therefore, Star 1 is larger in radius than Star 2.
This is a true statement
Two stars are on the main sequence. Star 1 is spectral type B2, while Star 2 is spectral type K0. Therefore, Star 1 has a greater luminosity than Star 2.
This is a true statement.
Two stars are on the main sequence. Star 1 is spectral type B2, while Star 2 is spectral type K0. Therefore, Star 1 is larger in mass than Star 2.
This is a true statement.
What makes the discovery of Mayor and Queloz (51 Peg) so important?
This was the first hot Jupiter exoplanet discovered around a Sun-like star.
Which of the following are three reasons the Orion molecular cloud is a useful "laboratory" for studying the stages of star formation?
We know the distances to Orion's star-forming regions, so we can determine the luminosity of objects in the region. The Orion Nebula is a relatively nearby stellar "nursery" so we have a close-up view of star formation processes. We can observe several stages of the star formation process in stars and protoplanetary disks in the region.
What is the role of hydrogen in stellar evolution?Describe its overall importance.
When a star exhausts the hydrogen in its core, it leaves the main sequence and begins to fuse hydrogen in a shell outside the core. The core increases in mass as the shell produces more helium.
_____ stars are held up by electron degeneracy.
White dwarf
A spectroscopic binary is...
a binary star in which the components are not seen separately, but whose binary nature is indicated by periodic variations in radial velocity (changes in the Doppler shift of the spectral lines), indicating orbital motion.
A type Ia supernova requires....
a binary system with a white dwarf star.
What is a pulsar?
a rapidly rotating neutron star.
Which of the following is closest in size (radius) to a neutron star?
a small city
Line broadening allows the determination of ....
a star's rotation rate.
Main sequence stars....
are luminosity class V.
Williamina Fleming
arranged spectral sequence based on strength of the H line
Annie Jump Cannon
arranged spectral sequence based on temperature
Any object, regardless of its mass, from which light cannot escape is called a.....
black hole.
How do we know that sunspots are associated with strong magnetic fields?
by observing the Zeeman effect
For a Sun-sized star, ...
core fusion will stop when a carbon core is formed.
From the center outward, which of the following lists the "layers" of the Sun in the correct order?
core, radiation zone, convection zone, photosphere, chromosphere, corona
The three types of binary stars are...
eclipsing spectroscopic visual
True/False: You can determine the space velocity of a star from its radial velocity only?
false
Suppose you and I have very accurate atomic clocks. We synchronize the clocks, then you go on a balloon ride up to about 100,000 ft above the Earth's surface. Compared to my clock, your clock runs....
faster.
James Lick
financed building the worlds largest telescope of the time
Black holes form....
from stars of very high mass.
Which of the following is the best candidate to explain cosmic rays?
galactic or extra-galactic supernova explosions
At the center of the Sun, fusion converts hydrogen into
helium, energy, and neutrinos
Edward Pickering
hired the Harvard computers
A large exoplanet that has a fast orbit near enough to its host star so that the planet is heated (like that of 51 Peg) is called a ....
hot Jupiter.
The red giant stage occurs when...
hydrogen shell burning begins causing the star to expand, become more luminous, and have a cooler surface temperature.
The Chandrasekhar limit...
is the maximum mass of a white dwarf star.
A star is no longer considered a main sequence star when....
it stops fusing hydrogen in its core.
Compared to the Sun a Red Giant star is....
larger in radius higher in surface temperature higher in luminosity
Which of the following stages of a star's life will last the longest?
main sequence
The position of a star along the main sequence is determined by its _____.
mass
What is the primary factor that determines how long it takes a star to form?
mass
What is the single factor that determines how long a star will remain in the main sequence stage?
mass
The hottest temperatures observed in the ISM are measured in ____ of degrees.
millions
Hydrogen fusion in the sun requires a temperature (in Kelvin) of
millions of degrees
Most of the energy of a supernova is released in the form of..
neutrinos.
If astronomers determine the mass of an unseen companion in a binary star system to be 2.1 solar-masses, then this is most likely a ......
neutron star
The maximum mass for a neutron star is....
not known exactly but is somewhere about 3 solar-masses
How do human-built nuclear power plants on Earth generate energy?
nuclear fission
The life story of a star can best be understood as a competition between two forces. When the forces are in balance, the star is said to be in a state of hydrostatic equilibrium. What are the two forces?
nuclear pressure gravity
A main sequence star is...
one that is converting hydrogen into helium in its core.
Which layer of the Sun do we normally see with out untrained eye?
photosphere
Which of the following stages is best explained as the outer layers of a star blown into space and illuminated by a hot central star?
planetary nebula
What state of matter is the Sun?
plasma
Which of the following cannot be determined from the spectrum of a star?
proper motion
the difference between a red giant and a red supergiant is....
red supergiants were massive main sequence stars.
Which of the following does general relativity NOT predict?
snow in July
If star A has an apparent magnitude of 1.0 and star B has an apparent magnitude of 0.2, which would appear brighter?
star B
One AU is ....
the average distance from Earth to the Sun
In protostar formation, as a molecular cloud collapses it begins to spin faster and flatten. This is due to....
the conservation of angular momentum.
What is the name of the region around a star where an orbiting planet would have a surface temperature capable of sustaining water in the liquid state?
the habitable zone.
The event horizon is....
the surface of a black hole defined by the Schwarzschild radius.
A disk of gas and dust appears to be an essential part of star formation.
true
True/False: Most stars have compositions very similar to that of the Sun.
true
A star of spectral class B2 undergoes core collapse and explodes in a supernova. What type of supernova is this?
type II
What happens after the electron degenerate iron core of a high mass star collapses into a neutron degenerate core?
type II supernova
Which of the following (more than one) enriches the ISM with elements beyond iron?
type II supernova type Ia supernova
SN 1987A is an example of a....
type II supernova.
Distances within the solar system are most accurately determined by ....
using reflected radar waves from solid objects and Kepler's laws for nonsolid objects.
What will be at the end of the Sun's stellar evolutionary track?
white dwarf
A star with surface temperature of 28,500 K and low luminosity would be a....
white dwarf.
If you observe a star of magnitude 14, then...
you are using a telescope