Astrophysics Part 2 : Classification Of Stars ( With Distances And Magnitude)

Question 1

What is apparent magnitude and absolute magnitude?
Also what is the equation associated with it?

Answer 1

Apparent magnitude (m) - how bright the star appears from Earth
Absolute magnitude (M) - how bright the star would appear if it were placed 10 parsecs from earth.

m - M = 5log(d/10)

Question 2

What is the Hipparcos scale? Also what is the difference in intensity of one magnitude?

Answer 2

The Greek astronomer Hipparchus catalogued stars, defining their brightness in terms of apparent magnitude (m), with the brightest stars a magnitude of 1 and the faintest a magnitude of 6.
It has since been extended to include brighter objects like the sun, with an m of -26.47.

Magnitude 1 star is about 2.51 times brighter than a magnitude 2 star.

Remember that brightness is a subjective scale of measurement.

Question 3

Define parsec

Answer 3

The distance to an object that subtends an angle of one arc second (1/3600th of a degree) to 1 AU. (Image in a triangle where adjacent side is 1 parsecs away and the opposite side is 1AU).

Question 4

Define light year

Answer 4

Distance travelled by light in a vacuum in one year.

Question 5

What is the luminosity of a star?

Answer 5

The total energy emitted in the form of EM radiation each second.

Question 6

What is the intensity of an object we observe?

Answer 6

The power received from it per unit area at earth. This is the effective brightness of an object.

Question 7

What does the brightness of a star depend on?

Answer 7

It’s luminosity and distance from us. So the brightest stars will have a high luminosity and closest to us.

Question 8

How do I calculate the brightness (intensity) ratio between two stars?

Answer 8

IA / IB = 2.51^(mB - mA)
m = apparent magnitude oeither star 1 or 2

Question 9

What is astronomical unit defined as?

Answer 9

mean distance between earth and the sun

Question 10

What is stefans law and state what the values are?

Answer 10

The power output of a star (luminosity) is directly proportional to its surface area and its (absolute temperature)^4.

P = σAT^4

A = surface area
T = temperature in kelvin
σ = stefan constant

Used to compare the power output, temperature and size of stars.

Question 11

State Wien’s displacement law

Answer 11

The wavelength of a stars’s emission at peak intensity is inversely proportional to is absolute temperature.

λmaxT = constant = 2.9. 10^-3 mk

mk = metre-kelvin not millikelvin

Equation can be used to calculate either max wavelength of temperature on black body radiation curves.

Question 12

How do we calculate intensity of the radiation received from a star’s emission from earth?

Answer 12

I = P/4πd^2

P = power output of star in watts
d = distance from the star in metres

Question 13

What is a black body?

Answer 13

A black body absorbs electromagnetic radiation of all wavelengths and can emit electromagnetic radiation of all wavelengths.
It does not reflect any radiation - it absorbs all radiation incident on it.
A star should assumed to be a black body

Question 14

Draw 3 black body radiation curves for 3 black bodies of decreasing temperature.

Answer 14

Graph of intensity (power radiated) against wavelength. Check cgp or pmt

Question 15

Where is the sun located on the Hertzsprung-Russell diagram?

Answer 15

It’s a main sequence star and its spectral class is G, absolute magnitude is 4.83. Therefore it can be located on the diagram.

Question 16

What is the evolutionary path of a sun-like star (a main sequence star) on a HR diagram?

Draw it

Answer 16

Once main sequence star uses up all the hydrogen in its core, it will up to the red giant section. (Brighter and cooler)

Once red giant uses up all the helium in its core. It will eject its outer layers and will move down to the left on the HR diagram as it becomes a white dwarf. (Hotter and dimmer than a main sequence star).

see a diagram on pmt

Temp is logarithmic

Question 17

Draw an HR diagram with axis

Answer 17

Check cap or pmt
Axis:
absolute magnitude 15 - -10
Temperature 50000k - 2500k or OBAFGKM

Question 18

For a hydrogen absorption line to occur in the visible part of the star;s spectrum what energy level must the hydrogen atom electrons need to be in?

Answer 18

n = 2

Question 19

How is temperature related to absorption spectra?

Answer 19

High temperatures, where collisions between atoms give the electrons extra energy.
If the temperature is too high, though the majority of the electrons will reach n = 3 level or above instead os not so many balmer transitions

Question 20

A star has an intrinsic colour of yellow white. What is its spectral class, temperature and prominent absorption line?

Answer 20

G
5000 - 6000
Ionized and neutral metals

Question 21

A star has a temperature of 7500 - 11000. What is its spectral class, intrinsic colour and prominent absorption line?

Answer 21

A
Blue - white
H (strongest ionized metals)

Question 22

A star has a spectral class of O. What is its intrinsic colour, temperature and prominent absorption line?

Answer 22

Blue
25000 - 50000
He+, He, H

Question 23

A star has prominent absorption lines of He and H. What is its temperature range, intrinsic colour and spectral class?

Answer 23

11000 - 25000 k
Blue
B

Question 24

A star has a spectral class of M. What is its intrinsic colour, temperature range and prominent absorption lines?

Answer 24

Red
<3500
Neutral atoms,TiO

Question 25

A star has an intrinsic colour of White. What is its spectral class, temperature range and prominent absorption lines?

Answer 25

F 6000 - 7500 Ionised metals

Question 26

A star has a temperature range of 3500 - 5000k. What is its spectral class, intrinsic colour and prominent absorption lines?

Answer 26

K Orange Neutral metals

Question 27

Define one solar mass.

Answer 27

The mass of the sun ( 2 x 10^30 kg )

Question 28

What is a defining property of a supernovae?

Answer 28

Rapid increase in absolute magnitude.

Question 29

What is a defyning property of a neutron star?

Answer 29

Incredibly dense (the density of nuclear matter) When the core of a large star collapses, the gravity is so strong that it forces protons and electrons together to form neutrons.

Question 30

What is a defyning property of a black hole?

Answer 30

The event horizon of a black hole is the point at which the escape velocity becomes greater than the speed of light.

Question 31

what does the collapse of a red super giant cause?

Answer 31

Gamma ray bursts to form neutron stars or black holes

Question 32

What is a binary system?

Answer 32

Two stars orbiting a common centre of mass

Question 33

What is a type 1 supernovae? And how does it form a type 1a supernovae?

Answer 33

When a star accumulates matter from its companion star in a binary system and explodes after reaching a critical mass. A type 1a supernovae is a type 1 supernovae with a white dwarf. When the companion star in the binary system runs out of hydrogen, it expands, allowing the white dwarf to begin accumulating some of its mass. when the white dwarf reaches a critical mass, fusion begins and becomes unstoppable as the mass continues to increase, eventually causing the white dwarf to explode in a supernova.

Question 34

Astronomers use type1a supernovas as standard candles to calculate distances to far-off galaxies. what does a light curve for a typical 1a supernova look like?

Answer 34

Y axis - Absolute magnitude X axis - Time/days Check pmt Conventionally time is measured from the peak.

Question 35

Why do scientists believe that supermassive black holes are at the centre of galaxies?

Answer 35

This is because stars and gas near the centre of galaxies appear to be orbiting very quickly.They concluded there must be a supermassive object at the centre with a very strong gravitational field attracting them.

Question 36

What is dark energy and why is it controversial?

Answer 36

Reason behind the universe accelerating Has an overall repulsive effect throughout the whole universe. Since gravity follows the inverse square law it decreases with distance but dark energy remains constant throughout the whole universe. Has a greater effect than gravity and is therefore causing expansion speed to increase. Controversial. Because there is evidence for its existence, but no one knows what it is or what is causing it.

Question 37

What is the schwarzchild radius and how can it be calculated?

Answer 37

Radius of the event horizon, and can be calculated by using the formula in data booklet.