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Flashcards in Astrophysics Deck (72):

When are Absorption spectra produced.

Absorption spectra are produced when light, or more generally electromagnetic radiation, has been absorbed by matter. Consists of a continuous spectrum crossed by dark lines or bands.


What are active optics

Active optics describe a technique used to compensate for imperfections in the shape of telescope mirrors.


What are adaptive optics?

Adaptive optics is a technique used in telescopes that involves measuring and compensating in real time for atmospheric effects.


Define Angular resolution?

Angular resolution is measured in terms of the angle that is subtended by the object at the telescope. It is measured in radians, but more commonly in seconds of arc (arc seconds).


What is Apparent magnitude?

Apparent magnitude is the magnitude as measured by an observer on earth.


Define the Arc second?

Arc seconds are an angular measurement. There are 3600 arc seconds in 1 degree.


What is a Astrometric binary?

Astrometric binary is a binary star in which one component is too faint to be observed directly. The presence of the unseen component is inferred from the perturbation of the motion of the visible component.


Define Astronomy

Astrometry is the science of the measurement of the positions of stars and other celestial objects.


What is Astronomical unit (au)

The average distance from the earth to the sun.


Define Binary stars?

A double star system in which two stars orbit around their common centre of mass under the influence of their gravitational attraction.


Black hole

A star that has collapsed under its own gravitation to such an extent that its gravitational field is so intense that even light cannot escape from its surface.


Define Brightness?

Brightness in astrophysics is the intensity of light or other radiation emitted or received from a celestial body. Brightness is determined by the stars's size (radius) its surface temperature and its distance from us.


Name three things that affect the brightness of a star?

its radius surface temperature distance from us


What are Cepheid variables?

Cepheid variables are a class of pulsating stars which vary periodically in brightness.


what is the CNO cycle.

The carbon-nitrogen (CNO) cycle is a chain of nuclear fusion reactions by which energy may be generated in stars. The overall result is to transform hydrogen into helium. Carbon acts as a catalyst and nitrogen and oxygen are produced as intermediates.


Define Absolute magnitude?

Absolute magnitude is the magnitude a star would have if it were 10 parsecs from earth. It is the measure of the actual brightness, not the apparent brightness.


what is the colour index?

The difference between the magnitudes measured using a blue filter and the magnitude using a yellow filter, that is CI = B-V


Continuous spectrum

A complete spread of wavelengths.from red to violet.


What is degenerate matter?

Degenerate matter is highly dense form of matter that can exert a pressure as a result of certain quantum-mechanical effects. This pressure stabilises a white dwarf against the gravitational force.


what is diffraction grating?

Is a device for producing a spectrum of white light. It consists of a series of straight parallel lines scratched on an optically flat surface. Simliar but different to a prism.


What is an eclipsing binary star?

a binary star system in which the orbital planes ares seen almost edge-on so that eclipses occur. Thus causes the total brightness of the pair to vary periodically as each star passes in front of the other, the deeper minimum corresponding to the eclipse of the brighter star.


Describe fusion.

Fusion is the nuclear process by which light nuclei join together to produce a heavier nucleus. The process of energy production in stars.


Describe a galaxy?

A group of stars, dust and gas bound together gravtionally. Gallaxies typically contain billions of stars.


What are giant stars and where are they found on a HR diagram?

Giant stars are large, highly luminous stars which are brighter than main sequence stars of the same colour. Giants represent a late phase in stellar evolution. They are found above main sequence stars on a HR diagram.


Define a global cluster?

A spherical system of old stars usually found in the halo of a galaxy. Typically containing about 100 00 stars with very little dust or gas. They contain lots of evolved stars like red giants and white dwarfs.


Describe ground based astronomy.

Is the use of telescopes and other devices based on Earth. suffers from atmospheric problems including seeing and the absorption of most wavelengths, with the exception of visible and radio waves and some narrow windows in the near infrared.


Describe Helium burning.

Helium burning occurs when in a main sequence star hydrogen is burnt producing helium in the core. In time a core of helium is formed and a hydrogen-burning shell moves out from the core. When this helium core reaches 12% of the total mass, the core collapses and increases in temperature, the outer envelope expands and cools quickly. Helium burns to form carbon in the triple alpha process.


What is the triple alpha process?

The burning of helium to produce carbon. 3He fuse to give carbon + energy


What is a helium flash?

It is the explosive onset of helium burning in the degenerate core of an evolved star.


Describe the Hertzsprung gap.

A region on the H-R diagram to the right of the main sequence in which few stars are found. This is due to the stars rapid evolution away from the main sequence through this region.


Describe a Hertzsprung- Russell diagram.

A type of diagram that displays the brightness of stars versus their colour or spectral class or surface temperature.


What do H-R diagrams tell us?

The H-R diagram is important in the determination of the age, distance and evolutionary history of a star system.


What is interferometry?

A technique used to study optical or radio-wave interference. The device in which it occurs is called an interferometer. Signals from two or more telescopes are added together and produce interference patterns. 

Think Michelson morley experiement but with telesopes.


Describe Iron Core collapse.

For stars with inital masses greater than 8 solar masses, nuclear fusion contiues until iron is synthesised. The iron core doesn't iginite (Fe is very stable), gravity overhelms the star and the core collapses to radius of 20km. At this stage it stops collapsing (now too dense) and bounces back out in a violent explosion (releasing enormus amount of energy) which blows away the surrounding envelope. Asingle supernova can result in a single star shining brighter than a billion stars. 


What is a light curve?

A graph on which the magnitude of a variable star is plotted as a function of time. 


What is a line emission spectrum?

For an element this consists of lines of certain frequencies (colours) only. 


Define luminosity?

Luminosity is a measure of the actual brightness of an astronomical object. It is a measure of the power emitted by the object and is measured in watts. 


Define Magnitude?

What can we use it for?

Magnitude is a way of expressing the brightness of an astronomical object. Each magnitude is 2.5 times different from the magnitudes on either side of it. The smaller the magnitude, the brighter the object.

The magnitude of a star can be used to deduce the distanxce to the star. 


Describe a main sequence star.

Main sequence is a region on the H-R diagram containing the majority of stars. It is in this region that stars spend the main part of their lives converting hydrogen into helium via fusion. 


What is a Nubula?

They are a cloud of gas and dust that can be observed as either a luminous patch of light if a bright nebula, or a dark hole or band against a bright background if a dark nebula. Radio telescopes pick these up well. 


What is a neutron star?

A star at the end of its evolution. The core wasn't big enough to form a black hole. Their mass is similar to that of the sun but their diameter is only about 20km.


What is an open cluster?

Open clusters are galaxies containing a few hundred to a few thousand stars. Individual stars in the cluster are easily distinguished. They contain young stars (population 1 stars) with a relatively high percentage of metals.


Describe Parallax.

Parallax is the apparent movement of an object against a background when viewed from different positions.


Define parsec.

Parsec is the distance at which the radius of the earths orbit around the sun subtends an angle of one sec of arc. 


Describe Period luminosity relationship.

What can it be used for?

For Cepheid variables there is a direct relationship between period of the variation in their brightness and their luminosity.

It can be used to determine the ditance to the star cluster containing the variable. 


What is photometry?

Photometry is the measurement of light intensity especially that from distance stars. 




What is 2?

 4 - did you really have to check the answer!


First to read this gets a choclate. 


when do plantetary nubulae occur?

They occur where a shell of gas expands outwards from a star in the later stages of its evolution between the red giant and white dwarf stages.


What are population 1 and 11 stars?

Theya re classes of the stars of different ages. Population 11 stars were formed early in the history of the galaxy. They are generally older and have a smaller content of heavy elements. Population 1 stars are younger and are associated with intersellar gas and dust. 


Decribe the proton -proton cycle.

A series of nuclear fusion reactions by which stars generate energy. The overall effect is convert 4 hydrogen nuclei into 1 helium nuclei.

Since the temperature required is about 10 million kelvin, it is believed to be the major source of energy in the sun and all main seuence stars cooler than the sun. The carbon cycle prodominates in hot stars. 


What is a protostar?

A protostar is a newly fromed star in which nuclear reactions have not yet started.


What is a pulsar?

is another name for a neutron star.


What is a Quasar?

Quasars are distant starlike sources, that is , point sources that exhibit a strong red shift. Tjey emit enormous amounts of energy.


Describe red shift

Red shift is the shift of the spectral lines from a receding light source towards the red end of the spectrum.


What is resolution?

Resolution is the measure of a telescopes ability to distinguish between two very close objects.


What is 'seeing' ?

The distortion of the image of a distant light source by the earths atmosphere.


What is sensitivity?

In astrophysics, the light collecting ability of a telescope. The bigger the diameter of the collecting surface, the more light it can collect.


Spectra and what information can they tell us?

Stars can be classified according to their spectra, which can be emission or absorption spectra and are detected by spectroscopes. The spectra of stellar objects allows use to determine a lot of information about the object such as its surface temperature and chemical composition.


Spectral class

is a classification scheme for stars based on the spectrum of the satrs. Goes from hot blue O stars through B,A,F,G,K to the cool red M stars (O Be A Fine Girl Kiss Me). Each group is divided into 9 others designated by a number. Our sun is a G@ star.


What is a spectroscopes?

are optical devices used to disperse light from a source into its spectrum. Uses prsims or diffraction gratings to disperse the light. Can be transmission or reflecting types.


What is a spectroscopic binary?

A binary star which is recognised by the periodic doubling of its spectral lines.


Describe Spectroscopic parallax

is a method using spectra to determine the distances to stellar objects. It does not actually use parallax. 

From an objects observed spectrum, astronomers can deduce its spectral class and luminosity. This allows them to determine its location on the H-R diagram and from this, its absolute magnitude. 

This in conjunction with its apparent magnitude, measured by photometry, allows the distance tob calculated.


Stellar equilibrium

(How a star maintains its size)

is the balance between the forces of gravity causing a star to collapse and the outward forces due to the energy released in nuclear reactions.


Stellar evolution

is the cycle in which stars are born, mature and eventually die. The way in which stars evolve is determined by their mass. The greater the mass, the faster they evolve. The evoltion of stars can be tracked on a H-R diagram.


What is a Supernovae?

are the end result of a massive star which explodes and increases in brightness by 1 billion times or more. In the explosion, the heavy elements (heavier than iron) are formed.


Trigonmetric parallax

What are the limitations?

is the method used to determine the distance to the nearest stars.

It is limited by the size of the baseline that is set by the diameter of the earths orbit around the sun.

Also by 'seeing' if taken from earth surface.


Turn off point

The point on the H-R diagram where stars leave the main sequence to become red giants. In general, the lower this point, the older the cluster.


What are variable stars?

are stars that undergo significant variation in its luminosity.

Can be intrinsic variables, where varaiblility is caused by changes in the physical properties of the stars themselves or extrinsic variables where variablity is caused by external properties like rotation or eclipses.


Visual binary

A binary star whose indivudal components can be distinguished through a telescope.


White dwarf

occur at the end of a stars evolution. It has a mass about equal to that of the sun and a diameter about that of the earth. No nuclear processes are continuing and it eventually ends up as a cold black dwarf.


Zero-age-main-sequence (ZAMS)

is the lower border of the main sequence of a H-R diagram