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

Describe the Hipparcos scale

Answer 8

A logarithmic scale to measure the magnitude of brightness of stars. A increase of 5 is a division of 100. Smaller number represent bigger magnitudes.

Question 9

What is apparent magnitude?

Answer 9

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

Question 10

What is the absolute magnitude of a star?

Answer 10

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

Question 11

What is d measured in for the magnitude equation?

Answer 11

Parsecs

Question 12

What is a blackbody?

Answer 12

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

Question 13

Describe the blackbody radiation curve

Answer 13

y-axis is power radiated at each wavelength. 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. Higher temperature blackbodies have a higher peak and a shorter peak wavelength.

Question 14

Why can we assume a star is a blackbody?

Answer 14

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 15

What is Wien’s displacement law?

Answer 15

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

Question 16

What is the photosphere of a star?

Answer 16

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

Question 17

What is Stefan’s law

Answer 17

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

Question 18

Order of class of stars from hottest to coolest

Answer 18

OBAFGKM

Question 19

Recite table of class of stars

Answer 19

See grid on specification

Question 20

Why do spectra from stars contain absorption lines?

Answer 20

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 21

What are Hydrogen Balmer lines?

Answer 21

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 22

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

Answer 22

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

Question 23

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

Answer 23

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

Question 24

Describe the Hertzsprung-Russel diagram

Answer 24

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 25

Describe the evolution of a star

Answer 25

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 26

How does a star form?

Answer 26

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 27

Main sequence stage

Answer 27

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 28

Red giant stage

Answer 28

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 29

White dwarf stage.

Answer 29

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 30

Death of high mass star stage

Answer 30

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 31

Typical supernova characteristics

Answer 31

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 32

Type 1a supernova

Answer 32

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 33

How did type 1a supernovae lead to controversy?

Answer 33

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 34

Describe neutron stars

Answer 34

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 35

What are pulsars?

Answer 35

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

Question 36

Black hole stage

Answer 36

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 37

Describe a black hole

Answer 37

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 38

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

Answer 38

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

Question 39

Supermassive black holes

Answer 39

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 40

Gamma ray bursts

Answer 40

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.