Chapter 12 Flashcards
What are some properties of stars?
- Luminosity
- Surface Temperature
- Mass
What do stars form?
Stars form the gravitational collapse of clouds (nebula)
What are stars composed of?
~ 75% hydrogen
~ 25% helium
< 2% heavier elements
What is Luminosity?
Total amount of energy per second that a star radiates (emits into space)
- Luminosity of our sun is
3. 8x10^26 watts (joules/s)
What is Apparent Brightness?
Total amount of energy per second per area that reaches us from a star
Luminosity vs. Apparent Brightness
- Luminosity is a constant property of a star
- Apparent Brightness of a star depends on distance, the farther away the smaller the AB
What is the formula for Apparent Brightness?
AB= Luminosity/4(pie)d^2
d=distance between star and Earth
What are the Luminosity Range of Stars?
10^6 to 10^-4 (1,000,000 to 0.0001)
What is Parallax?
The shift in apparent position of an object in the sky when seen from different positions due to relative proximity
How can we measure distance through stellar parallax?
If the star is sufficiently close, we can measure its change of position with respect to distant stars as the Earth orbits the Sun. (Smaller the distance, larger parallax angle)
What is a parsec?
Distance of an object that presents a stellar parallax of 1 arc sec.
1 parsec=3.26 light years
What is the classification of Apparent Magnitude?
Used with whole numbers from 1-6.
1 being stars with largest AM and 6 being stars with smallest AM that is visible to the naked eye.
What is the formula for Apparent Magnitude?
m= -2.5*log10(ApparentBrightness)+K
K= -11.49 in CGS units [cm/g/s]
What is Absolute Magnitude?
Apparent Magnitude of a star if it were placed at a distance of 10 parsecs from Earth
What is the formula for Absolute Magnitude?
M= -2.5log10 (Luminosity/4(pie)10pc^2)+ K
What is Stellar Temperature?
Refers to the temperature of a star’s surface.
Sun’s Stellar Temperature= 5,800 K
How can we measure the Stellar Temperature of a Star?
Through the frequencies of light it emits. The hotter a star, the higher the light frequencies. “Cold Stars” will emit infrared and appear red. “Hot Stars” will emit ultraviolet and appear violet. We estimate a star’s stellar temperature by its color.
What is the Stellar Classification based on Stellar Spectra from hotter to cooler?
O-stars: >30,000 K B-stars: 30,000-10,000 K A-stars: 10,000-7,500 K F-stars: 7,500-6,000 K G-stars: 6,000-5,000 K (our sun) K-stars: 5,000-3,500 K M-stars: <3,500 K
What are Binary Stellar Systems?
Systems composed of 2 stars. Common in our galaxy (about 1/2 of stars in our galaxy are found in binary systems)
What are visual binary systems?
Systems where we can directly see both stars
What is Eclipsing Binary Systems?
Systems where we can detect a change in apparent brightness during eclipses
What is Spectroscopic Binary Systems?
Systems that are detected through Doppler Shifts of the spectral lines as the stars orbit each other.
- When spectral line increases frequency (blue shifted)
- When spectral line decreases frequency (red shift)
What is Stellar Mass?
Stellar Masses im binary systems can be determined through Newton’s Law of Gravitation if we can measure their period “P” and one of:
A) distance between the 2 stars or
B) orbital velocities
P^2=(4(pie)^2/G(M1+M2)a^3
What are some patterns in Stellar Parameters?
- Horizontal axis: Surface temperature
- Vertical axis: Stellar Luminosity
- Stars in the upper right are called “Supergiants” and “Giants” because their size is larger and more luminous
- Stars in the lower left are called “white dwarfs” because their size is smaller and less luminous
What are the Stellar Luminosity Classes?
I- Supergiants II- Bright giants III- Giants IV- Subgiants V- Main Sequence
What are the Stellar Spectral Classes from hotter to cooler?
- O0…O9
- B0…B9
- A0…A9
- F0…F9
- G0…G9
- K0…K9
- M0…M9
What does “G2 V” star mean?
That means our Sun has a spectral class of G2 and it is a main sequence star
What are main sequence stars?
They are like our Sun, having hydrogen burning nuclear core
Main Sequence Stars: Mass
- Stars in main sequence are in a stable balance between pressure from the core and gravitational forces.
- Larger the mass, stronger the gravitational forces, greater the pressures, higher the rate of nuclear reactions, the brighter the star.
- Mass of star will determine its luminosity and size
- Surface temperature depends on the size of the star
- Mass will determine the position of a star in main sequence
- The more luminous the star, the shorter the lifetimes will be
What are giant stars?
Are stars that exhausted their hydrogen nuclear fuel in the core, pressure-gravity balance is lost and the stars expand
What are white dwarfs?
- They are about the size of Earth but with a mass like the Sun.
- “Leftover” cores of giants
- avoid further gravitational collapsing through pressure of “degenerate matter”
- emit radiation due to their hot surface temperatures
What are star clusters?
Stars that are believed to be formed from the same initial interstellar cloud
Where are stars formed?
Stars are formed from clouds of interstellar gas
What properties follow in a star cluster?
1) The stars are approximately at the same distance from Earth
2) The stars in a star cluster are approximately the same age
What are the two types of Star Clusters?
Open and Globular Clusters
What is the open cluster?
Open Cluster (Pleiades):
Contains several thousand stars and are within a space of ~30 light years across. Are seen within the disk of our galaxy. Tend to be relatively young (several millions years)
What is the Globular Cluster?
Globular Cluster (M 80):
Contains several of million stars and are within a space of ~60-150 light years across. Typically seen in the halo of our galaxy (halo is the space above or below our galaxy disk). Tend to be relatively old (several billion years)
How do we know the age of a Star Cluster?
The lifetime of a star in the main sequence depends on the spectral type. The point on the H-R diagram where the stars of a star cluster diverge allows us to estimate the age of the cluster.
