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Astrophysics > Astrophysics > Flashcards

Astrophysics Flashcards

1
Q

Binary star

A

Two stars orbiting a common point

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2
Q

Black dwarf

A

The remnant of a white dwarf star after it has cooled down. It has low luminosity

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3
Q

Black hole

A

A singularity in space-time which is an end to the evolution of a super massive star

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4
Q

Brown dwarf

A

Gas and dust that did not reach high enough temperatures to initiate fusion. These objects continue to compact and cool down. “Failed star”

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5
Q

Cepheid variable

A

A star of variable luminosity. The luminosity increases sharply and falls off gently with a well-defined period. The period is related to the absolute luminosity of the star and so can be used to estimate the distance to the star.

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6
Q

Clusters of galaxies

A

Two or more galaxies positioned close enough that they begin to affect one another through gravitation.

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7
Q

Comet

A

A relatively small extraterrestrial body consisting of a frozen mass or dust that travels around the sun in a highly elliptical orbit

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8
Q

Constellation

A

A group of stars which are in a particular pattern or design. Some of which are given names by humans.

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9
Q

Cluster

A

Gravitationally bound system of galaxies/stars

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10
Q

Dark matter

A

Matter in galaxies that is too cold to radiate (does not give off electromagnetic radiation). Its existence is inferred from techniques (gravity) rather then direct visual contact

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11
Q

Galaxies

A

Giant assemblies of stars, gas, and dust held together by the gravitational forces they have on each other. Shapes can be spiral, globular or irregular. Our galaxy is called the Milky Way.

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12
Q

Interstellar medium

A

Gases and dust that are filling the space between stars. Interstellar mass’s density is very low with about one atom of gas for every cubic centimeter of space.

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13
Q

Main sequence star

A

A normal star that is undergoing nuclear fusion of hydrogen into helium. Our sun is a typical main sequence star.

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14
Q

Neutron star

A

A very dense star, consisting only of uncharged neutrons. They are created when very massive stars explode, leaving this neutron ‘ball’ behind. A neutron star is smaller then a white dwarf and extremely dense. It is microscopic and is a prime example of microscopic quantum physics.

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15
Q

Nebulae

A

From the Latin word for ‘cloud’. Used to label all sorts of stuff in space, that are now known as star cluster or galaxies. It is sometimes still used for a concentration of gas and dust

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16
Q

Nova

A

A sudden increase in luminosity of a white dwarf caused by material from a nearby star falling into a white dwarf

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17
Q

Parallax

A

The apparent motion of a star against the background of a more distant star, due to the motion of the Earth around the Sun. The angle is measured at different times during the year. The distance of the Sun to the Earth is known. Distances are specified in parallax angles in seconds of arc (parsec). At large distances the uncertainty becomes too large and it can’t be applied.

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18
Q

Planetary nebula

A

The ejected envelope of a red giant star consisting of an expanding shell of ionized gas.

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19
Q

Pulsar

A

The sending out of sharp, strong burst of radio waves at regular intervals ranging between milliseconds and 4 seconds. They appear to be rapidly rotating highly magnetic neutron stars. The pulses are very energetically charged particles. The rotation and pulse rates gradually slow down as energy is radiated away.

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20
Q

Quasar

A

Small, extraordinarily luminous extragalactic objects with high redshifts. They do not seem to conform to Hubble’s Law. They are as bright as nearby stars, but display very large redshifts. According to Hubble’s law the quasars must be either extremely distant and incredibly bright (thousands of times brighter than ordinary galaxies) or that they are closer than the redshift suggests. There is either an unresolved brightness problem or an unresolved redshift problem. One theory is that quasars could be powered by black holes.

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21
Q

Red dwarf

A

A very small star with a relatively low temperature, thus red in color

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22
Q

Red giant

A

Luminous stars with low surface temperature. These stars are produced when hydrogen in the core of the star has fused into more heavy helium. Gravity forces the star to contract, but at the same time it heats up. The hydrogen around the core now burns more fiercely and causes the outer envelope of the star to expand and thus cool. This low surface temperature produces light at a longer wavelength.

23
Q

Stellar cluster

A

A group of stars that are held together by each other’s gravity and so are orbiting around their centre of mass

24
Q

Supernova

A

Gigantic stellar explosions. These occur when very massive stars explode. Because of Einstein’s law, E=mc2, fusion can not take place after that iron is created. The star collapses since there is no force that can outweigh the gravitational force. When it is impossible to compress it further, it explodes violently giving out extremely luminous light.

25
Q

White dwarf

A

Stage in which a star has used up its helium and its outer layers escape into space, leaving behind a hot, dense core that contracts

26
Q

Solar system

A

The Sun is orbited by planets, comets, and asteroids

27
Q

Elliptical orbit

A

Non-circular path which has the star at one focus and is called eccentric. A body in an elliptical orbit changes speed as it travels.

28
Q

Astronomical unit (AU)

A

The mean distance from the center of the Earth to the center of the Sun. 1 AU is about 150 million meters.

29
Q

Light year (Ly)

A

The distance that light travels in one year. 1 Ly is about 63000 AU.

30
Q

Apparent motion of stars

A

In 24 hours, the stars appear to rotate about the extension of the Earth’s axis. Each day they move slightly forward at the same time of night. In one year, some stars move above and below the horizon due to the Earth’s tilt.

31
Q

Apparent

A

Something extraterrestrial as seen from Earth

32
Q

Nuclear fusion

A

The chemical reaction that goes on inside a star. Consists of protons fusing to make helium in a complex reaction which also produce positrons, neutrinos and gamma rays.

33
Q

Stable star

A

There is a balance between collapse due to gravitational force and expansion due to KE of particles (radiation pressure). The star’s source of energy keeps it stable.

34
Q

Luminosity (L)

A

The total amount of energy emitted by the star per second, relevant unit: Watts (joules per second). It depends on the star’s temperature and its size.

35
Q

Apparent brightness (b)

A

The amount of energy received per unit area. Unit: Watts over meters squared.

36
Q

Black body radiation

A

A star is approximately a perfect emitter of a continuous radiation spectrum which changes depending on the temperature of the body. It is usually plotted as wavelength vs intensity.

37
Q

Stefan-Boltzmann Law

A

For a black body, the total power per unit area (intensity) is proportional to the fourth power of the absolute temperature.

38
Q

Wien Law

A

For a black body spectrum, the most intense wavelength is inversely proportional to the absolute temperature.

39
Q

Absorption spectra

A

The continuous spectrum from a star includes dark absorption lines corresponding to elements in the star’s outer layers. This can be used to identify the elements. The relative strengths of the spectral lines can accurately predict the temperature of the star.

40
Q

Spectral classification

A

The Harvard system from hottest (60 000K) to coolest (2 000K) is O-B-A-F-G-K-M

41
Q

Red and blue shift

A

The wavelength of the light from stars which are moving towards or away from the viewer is shifted slightly in the blue (shorter wavelength) or red (longer) directions respectively.

42
Q

Hertzsprung-Russel Diagram

A

A graph which shows stars classified by color and luminosity. The regions of the H-R diagram are Main Sequence, Giants, Supergiants, White Dwarfs.

43
Q

Stellar parallax

A

A method of determining the distance to nearby stars. The method involves measuring the apparent shift of a star amongst the background of more distant stars as it is viewed through telescopes from positions in the earth’s orbit that are 6 months apart.

44
Q

Parsec (pc)

A

The distance to a point whose maximum parallax viewed from the Earth is one second of arc. 1 pc = 3.26 Ly = 210,000 AU

45
Q

Apparent magnitude (m)

A

A measure of how bright a star appears from Earth.

46
Q

Absolute magnitude (M)

A

The magnitude of a star viewed from a distance of 10 pc.

47
Q

Spectroscopic parallax

A

Method of measuring distance to stars which are too far away for stellar parallax. It combines Wien’s Law, the HR diagram and apparent brightness.

48
Q

Newton’s model of the Universe

A

This assumes that the universe is infinitely old, infinitely large, static and uniform.

49
Q

Olber’s paradox

A

According to the Newtonian model, there should be stars in every direction one looks, so what is the night sky not bright?

50
Q

Expansion of the Universe

A

The light from galaxies is red-shifted, and even more so the further away they are. This implies that the universe is expanding.

51
Q

Big Bang model

A

From the speed of the receding galaxies, we can deduce that the universe originated from a single point 13.8 billion years ago. At the moment of the Big Bang, space and time came into existence.

52
Q

Cosmic microwave background radiation

A

The Big Bang model predicts that after 300 000 years at 4 000 K, atoms formed and the universe became transparent for the first time. The radiation which could now travel through it is still visible today as microwaves coming from all directions.

53
Q

Critical density

A

The density of the universe at which it will continue to expand forever at a steady rate.

54
Q

International astrophysics research

A

There are many projects which involve cooperating nations, such as the International Space Station and the Hubble Space Telescope.

Astrophysics (1 decks)

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