3.9.2 Classification of stars Flashcards
What is apparent magnitude?
The perceived brightness as seen from Earth
What is the Hipparcos scale?
A scale of apparent magnitude initially labelled 1.0-6.0, with 1.0 being the brightest, 6.0 being the dimmest
What is the relation between brightness and apparent magnitude?
In the Hipparcos scale, when apparent magnitude decreases by 1, brightness increases by 2.51 (100^(1/5))
What is a parsec?
The distance at which the radius of the Earth’s orbit (1AU) subtends an angle of 1 arcsecond
What is absolute magnitude?
The apparent magnitude of an object if it were placed 10pc away from Earth
What is Stefan’s law?
The output power of a black body radiator is directly proportional to its surface area and temperature^4
P=σAT^4.
What is Wien’s displacement law?
The peak wavelength of emitted radiation is inversely proportional to the absolute temperature of the object.
Tλ_max = constant
What is the general shape of black-body curves?
Peat at λ_max, then slopes down at the sides
What is assumed about a star when measuring its temperature (?)
Its a black body
What are the spectral classes?
O
B
A
F
G
K
M
What are Hydrogen Balmer lines?
Found in the spectra of O,B and A stars. They are caused by the excitation of hydrogen atoms from the n=2 state to higher/lower energy.
What is the parallax?
The apparent change of position of a nearer star in comparison to distant stars
How many light years are there in a parsec?
3.26
What are standard candles?
They are objects whose absolute magnitude is known and whose apparent magnitude can be measured.
What is a black body?
A body that absorbs all electromagnetic radiation of all wavelengths and can emit all wavelenghs of radiation
What is luminosity?
The total power output of a star (W)
What is the brightness of a star?
The intensity of radiation received on Earth by the star (Wm^-2)
What is the formula for the inverse square law of radiation?
I=P/(4πd^2)
For a given star, the luminosity is constant
What does the inverse square law of radiation assume?
The source is treated as a point
The power from the source radiates uniformly through space
No radiation is scattered between the source and Earth
What is AU?
Astronomical Unit: the average/mean distance between the centre of the Earth and the centre of the Sun
What is a light year?
The distance travelled by light in a year
What are the conversions for arcminutes and arcseconds?
1 degree = 60 arcminutes
1 arcminute = 60 arcseconds
What is the absolute magnitude relationship?
m-M=5log(d/10)
M=absolute magnitude
m=apparent magnitude
d=distance of star from Earth (pc)
What is the distance modulus?
m-M;
Used for determining the relative distance of a star.
Negative for stars closer than 10 pc
Positive for stars further away than 10 pc
Which spectral classes have the strongest Balmer lines?
A- strongest
B - second strongest
O - weakest
Describe the HR diagram
Supergiants at the top, giants at the top right, dwarves at the bottom, and main sequence stars forming a curve down the middle. Absolute magnitude on y-axis, 0 in the middle (corresponds to Sun), temperature on x-axis (40000K on left, 2500K on right)
What is a binary system?
One where two stars orbit a common mass
What are the 2 types of supernovae?
Type I; when a star accumulates matter from its companion star in a binary system and explodes after reaching a critical mass
Type II; The death of a high mass star after running out of fuel
How can supermassive black holes form?
The collapse of massive gas clouds while the galaxy was forming
A normal black hole that accumulated large amounts of matter over millions of years
Several normal black holes merging together
How did measurements not agree with Hubble’s law?
They found distant type 1a supernovae to be moving away from Earth at an increasing, speed, hence the universe is not expanding at a constant rate but accelerating.
Why are supernovae used as standard candles?
Supernovae (occur at same critical mass, so have similar peak absolute magnitude), producing consistent light curves.
