Stars

Question 1

What is one light year?

Answer 1

The distance light travels through space in one year

Question 2

What is parallax?

Answer 2

Where the position of nearby stars appears to shift against the background of more distant stars as the earth moves around its orbit

Question 3

What is one astronomical unit?

Answer 3

The mean distance between the centre of the sun and the centre of the earth

Question 4

What is the parallax angle?

Answer 4

The angle subtended to the star by the line between the sun and the earth. It is also half the angler shift of the star’s line of sight over 6 months. Generally measured in arc seconds.

Question 5

What is one arc second?

Answer 5

1 degree over 3600.

Question 6

What is one parsec?

Answer 6

The distance to a star which subtends an angle of one arc second to the line from the centre of the earth to the centre of the sun.

Question 7

What does a smaller parallax angle to a star mean?

Answer 7

The star is further away.

Question 8

What is a stars apparent magnitude and intensity dependent on?

Answer 8

• Its power output or luminosity
• Its distance from the earth

eg. for a constant luminosity, if the distance from the earth increases, the intensity of light decreases and therefore its apparent magnitude decreases.

Question 9

Describe the Hipparcos scale

Answer 9

A logarithmic scale to measure the apparent magnitude of brightness of stars. An increase of 5 in apparent magnitude is a scale factor of 100, so an increase in magnitude by n, represents a factor of 2.51^n. More negative numbers represent brighter stars.

Question 10

What is apparent magnitude, m?

Answer 10

A measure of brightness of a star that depends on the light intensity received from the star on earth.

Question 11

What is the absolute magnitude of a star, M?

Answer 11

The star’s apparent magnitude if it was 10pc from earth

Question 12

What is the equation relating absolute magnitude to the apparent magnitude?

Answer 12

m - M = 5 log_10 (d/10) , where m is apparent magnitude, M is absolute magnitude and d is distance in parsecs.

Question 13

What is a blackbody?

Answer 13

A perfect absorber of radiation at all wavelengths and therefore emits a continuous spectrum of wavelengths. Eg. a star

Question 14

What are the properties of a blackbody?

Answer 14

• A blackbody is a theoretical object
• A blackbody is defined as an object that absorbs all of the electromagnetic radiation that is incident on it.
• When in thermal equilibrium, it emits a characteristic distribution of wavelengths at a given temperature.
• It is therefore possible to deduce the temperature of a blackbody from the distribution of wavelengths emmited

Question 15

Describe the blackbody radiation curve

Answer 15

y-axis is relative intensity, x-axis is wavelength in micrometers. Visible range is about from 0.3 to 0.7 micrometers. Line curves up from near origin to peak wavelength then curves back down not as steep and levels off.

Question 16

Why can we assume a star is a blackbody?

Answer 16

Because any radiation incident on it would be absorbed and none would be reflected or transmitted by the star. Also, the spectrum of thermal radiation emitted by the star is a continuous spectrum with an intensity distribution that matches the shape of a blackbody radiation curve.

Question 17

What is Wien’s displacement law?

Answer 17

The wavelength at peak intensity is inversely proportional to the absolute temperature of the photosphere of a star.

Question 18

What is the photosphere of a star?

Answer 18

The light-emitting outer layer of a star. Sometimes called the surface of a star.

Question 19

What is Stefan’s law

Answer 19

The total energy per second emitted by a blackbody (luminosity or power output) is proportional to its surface area, and the absolute temperature of the photosphere of the star to the power 4.

L = σ 4 π r^2 T^4

Question 20

Order of class of stars from hottest to coolest

Answer 20

OBAFGKM

Question 21

Recite table of class of stars

Answer 21

See grid on specification

Question 22

Why do spectra from stars contain absorption lines?

Answer 22

There is an atmosphere of hot gases above the photosphere. Atoms, ions and molecules of these gases absorb light photons of certain wavelengths emitted from the photosphere. The light that passes through these gases is therefore deficient in these wavelengths so its spectrum contains absorption lines.

Question 23

What are Hydrogen Balmer lines?

Answer 23

The absorption lines on an emission spectrum from a star that correspond to excitation of hydrogen atoms from the n=2 energy levels to higher energy levels.

Question 24

Why are Hydrogen Balmer lines only found in the spectra of O, B and A stars?

Answer 24

Other star’s are not hot enough for excitation of hydrogen atoms, due to collisions, to the n=2 state.

Question 25

Why don’t hydrogen atoms in the n=1 state absorb visible photons?

Answer 25

Because visible photons don’t have enough energy to causes excitation from n=1.

Question 26

Describe the Hertzsprung-Russel diagram

Answer 26

y-axis is absolute magnitude ranging from +15 up to -10. x-axis is temperature or spectral class ranging from 50k to 2.5k or OBAFGKM. Sun(G,+5). Main sequence band class B to M magnitude -10 to +15 and is straight, then levels, then straight with same gradient as start. Red giants(G-M,0to-5). White dwarfs(B-F,+15to+10).

Question 27

Describe the evolution of a star

Answer 27

Protostar, main sequence star, red giant.
If low mass then white dwarf, invisible.
If high mass then supergiant, supernova, neutron star, black hole if mass of neutron star would be 3x mass of sun.

Question 28

How does a star form?

Answer 28

Dust and gas clouds in space contract under their own gravitational attraction becoming denser and denser to form a protostar. In the collapse GPE is converted into thermal energy as the atoms and molecules gain kinetic energy so the interior of the collapsing matter becomes hotter. If sufficient matter accretes to form the protostar, the temp at the core gets hot enough to do nuclear fusion. Otherwise it gradually cools. Energy from fusion of H increases core temp so more fusion occurs so outer layers heat and photosphere forms. Star.

Question 29

Main sequence stage

Answer 29

Newly formed star reaches internal equilibrium as the inward gravitational attraction is balanced by the outward radiation pressure. The star becomes stable at a constant luminosity. Star remains like this for most of its lifetime emitting light as a result of ‘hydrogen burning’ in its core.

Question 30

Red giant stage

Answer 30

When most H is converted to He, the core starts to collapse on itself and outer layers of star expand and cool. The temp of the He core increases as it collapses and causes surrounding H to form a ‘hydrogen burning’ shell which heats the core further. When core temp reaches 10^8K He can fuse. Luminosity increases.

Question 31

White dwarf stage.

Answer 31

When star stops doing fusion, it cools and core contracts causing outer layers to be thrown off as shells of hot gas and dust to form a planetary nebula around the star. The star is now little more than its core and is white hot due to release of gravitational energy. The contraction of core stops as electrons can’t be forced closer. Star is stable and is white dwarf which gradually cools as it radiates its thermal energy into space and eventually becomes invisible.

Question 32

Death of high mass star stage

Answer 32

Iron nuclei are most stable so no more fusion occurs. If the core mass is greater than 1.4 solar masses, the electrons in iron core can’t prevent collapse as they are forced to react with protons to form neutrons. This sudden collapse of core makes it more and more dense until the neutrons can’t get closer and it becomes rigid. The collapsing matter surrounding the core hits it and rebounds as a shock wave which propels the surrounding matter outwards into space in an explosion.

Question 33

Typical supernova characteristics

Answer 33

A sudden and very large increases in the luminosity of a star. Absolute magnitude of -15 to -20. Increase in luminosity occurs within about 24h. Luminosity gradually decreases over a number of years.

Question 34

Type 1a supernova

Answer 34

Reach peak luminosity about 10^9 times the sun. Strong absorption lines due to silicon. The absolute magnitude is known (-18 to -20) so they can be used to find distances between earth and its galaxy. Therefore they are standard candles.

Question 35

How did type 1a supernovae lead to controversy?

Answer 35

The distance from earth calculated was more than expected so the universe must be expanding at an accelerating rate which would require the input of energy. This lead to the theory of dark energy.

Question 36

Describe neutron stars

Answer 36

It is the core of a supernova after all the surrounding matter has been thrown off into space. Incredibly dense (similar to a nucleus). Contains only neutrons. Very small compared to sun.

Question 37

What are pulsars?

Answer 37

Pulsating radio stars. About 2 solar masses. Rapidly rotating neutron stars that produce beams of radio waves. How we discovered neutron stars.

Question 38

Black hole stage

Answer 38

If the core of a supernova is greater than 3 solar masses, the neutrons are unable to withstand the immense forces that are pushing them together. The core collapses in on itself and becomes so dense not even light can escape from it. The object is then a black hole.

Question 39

Describe a black hole

Answer 39

So dense not even light can escape from it because the escape velocity is greater than c. It can’t emit any photons and it absorbs any photons incident on it.

Question 40

What is the event horizon of a black hole and what is its radius called?

Answer 40

A sphere surrounding a black hole from which nothing can ever emerge. Schwarzschild radius.

Question 41

Supermassive black holes

Answer 41

Almost unimaginable mass thought to be at the centre of many galaxies. At the centre of a galaxy, star’s are much closer together than at the edges. A supermassive black hole at the centre could pull in millions and millions of stars and therefore gain enormous quantities of matter.

Question 42

Gamma ray bursts

Answer 42

Release vast amounts of energy in the form of jets of gamma radiation. They shoot out of the poles of supergiant stars when they collapse or fall into a black hole.

Question 43

How does increasing the temperature affect the shape of the blackbody curve?

Answer 43

Higher temperatures cause the peak of the intensity-wavelength curve to become sharper. The value of the peak wavelength also reduces as the temperature increases.

Question 44

Identify the object with the highest temperature for this graph of four blackbodies

Answer 44

Answer - A

Question 45

What is the ratio of intensities for two stars with given apparent magnitudes?

Answer 45

I1/I2 = 2.51^n, where n is the difference in apparent magnitude

Question 46

What is a stars luminosity also referred to as?

Answer 46

A stars power output, which is the total energy it emits each second.

Question 47

How are intensity and apparent magnitude affected by an increase in luminosity for a given distance?

Answer 47

As luminosity increases, intensity increases and apparent magnitude decreases

Question 48

How are intensity and apparent magnitude affected by an increase in distance for a given luminosity?

Answer 48

As distance increases, intensity decreases and apparent magnitude increases

Question 49

What do absolute and apparent magnitude depend on?

Answer 49

Absolute magnitude depends on luminosity whereas apparent magnitude depends on intensity

Question 50

How does the blackbody graph of a hotter star look like compared to a cooler star?

Answer 50

As the surface temperature of the star increases, it emits more radiation across all wavelengths and has a sharper peak, which occurs at a lower wavelength. So a hotter star appears bluer while a hotter star appears redder.

Question 51

What is the intensity of light incident on an object far away from a star?

Answer 51

where x is the distance from the star to the object and P is the stars power output or luminosity