3.9.2 Classification of stars

Question 1

What is apparent magnitude?

Answer 1

The perceived brightness as seen from Earth

Question 2

What is the Hipparcos scale?

Answer 2

A scale of apparent magnitude initially labelled 1.0-6.0, with 1.0 being the brightest, 6.0 being the dimmest

Question 3

What is the relation between brightness and apparent magnitude?

Answer 3

In the Hipparcos scale, when apparent magnitude decreases by 1, brightness increases by 2.51 (100^(1/5))

Question 4

What is a parcec?

Answer 4

The distance at which the radius of the Earth’s orbit (1AU) subtends an angle of 1 arcsecond

Question 5

What is absolute magnitude?

Answer 5

The apparent magnitude of an object if it were placed 10pc away from Earth

Question 6

What is Stefan’s law?

Answer 6

The output power of a black body radiator is directly proportional to its surface area and temperature^4
P=σAT^4.

Question 7

What is Wien’s displacement law?

Answer 7

The peak wavelength of emitted radiation is inversely proportional to the absolute temperature of the object.
Tλ_max = constant

Question 8

What is the general shape of black-body curves?

Answer 8

Peat at λ_max, then slopes down at the sides

Question 9

What is assumed about a star when measuring its temperature (?)

Answer 9

Its a black body

Question 10

What are the spectral classes?

Answer 10

O

B

A

F

G

K

M

Question 11

What are Hydrogen Balmer lines?

Answer 11

Found in the spectra of O,B and A stars

Question 12

What is the parallax?

Question 13

How many light years are there in a parsec?

Answer 13

3.26

Question 14

What are standard candles?

Answer 14

Supernovae (occur at same critical mass, so have similar peak absolute magnitude), producing consistent light curves.
They are objects whose absolute magnitude is known and whose apparent magnitude can be measured.

Question 15

What is a black body?

Answer 15

A body that absorbs all electromagnetic radiation of all wavelengths and can emit all wavelenghs of radiation

Question 16

What is luminosity?

Answer 16

The total power output of a star (W)

Question 17

What is the brightness of a star?

Answer 17

The intensity of radiation received on Earth by the star (Wm^-2)

Question 18

What is the formula for the inverse square law of radiation?

Answer 18

I=P/(4πd^2)
For a given star, the luminosity is constant

Question 19

What does the inverse square law of radiation assume?

Answer 19

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

Question 20

What is AU?

Answer 20

Astronomical Unit: the average/mean distance between the centre of the Earth and the centre of the Sun

Question 21

What is a light year?

Answer 21

The distance travelled by light in a year

Question 22

What are the conversions for arcminutes and arcseconds?

Answer 22

1 degree = 60 arcminutes
1 arcminute = 60 arcseconds

Question 23

What is the absolute magnitude relationship?

Answer 23

m-M=5log(d/10)

M=absolute magnitude
m=apparent magnitude
d=distance of star from Earth (pc)

Question 24

What is the distance modulus?

Answer 24

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

Question 25

Which spectral classes have the strongest Balmer lines?

Answer 25

B - strongest
A - second strongest
O - weakest

Question 26

Describe the HR diagram

Answer 26

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)

Question 27

What is a binary system?

Answer 27

One where two stars orbit a common mass

Question 28

What are the 2 types of supernovae?

Answer 28

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

Question 29

How can supermassive black holes form?

Answer 29

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

Question 30

How did measurements not agree with Hubble’s law?

Answer 30

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.