classification of stars

What is m?

Apparent magnitude: the perceived brightness of a star as seen from earth.

What is the Hipparcos scale?

The scale of apparent magnitude, with 1 being the brightest and 6 being the dimmest. This was then developed into the logarithmic scale where a magnitude 1 star is 100 times brighter than a magnitude 6 star.

What is the equation to compare the brightness of two objects A and B?

Intensity A/ intensity B= 2.51 ^ (apparent magnitude B - apparent magnitude A)

What is the luminosity of a star?

The total power output of radiation emitted by a star.

What is the brightness of a star defined as?

The intensity of radiation received on Earth from a star.

What is brightness equivalent to?

power per unit area, or light intensity and it’s measured in watts per metre squared.

What does the brightness of a star depend on?

How much light the star emits, how far away the star is.

What is the difference between luminosity and brightness?

luminosity tells us how bright the star is at its surface, brightness tells us how bright the star is as observed from earth.

What is the inverse square law of radiation?

I= L/ 4πd²

It assumes the source can be treated as a point, the power from the source radiates uniformly through space, no radiation is absorbed or scattered between the star and earth. L= luminosity, d= distance from earth to star

What does the inverse square law of radiation tell us?

For a given star the luminosity is constant, the intensity of the emitted light follows an inverse square law, for stars with the same luminosity the star with the greater apparent brightness is closer to the earth.

What is the astronomical unit (AU)?

The mean distance from the centre of the earth to the centre of the sun.

What is a light year?

The distance travelled by light in one year.

How has the apparent magnitude scale evolved due to telescopes?

The more negative the apparent magnitude, the brighter the object appears.

How can a light year be calculated?

distance = speed * time

where speed is speed of light and 1 year = 3.15 ×10^7 seconds.

What can angles smaller than 1 degree be measured in?

arcminutes or arcseconds, 1 degree= 60 arcminutes, 1 arcminutes= 60 arc seconds, 1 arc second= 1/3600 degree

What is a parsec?

The distance at which the radius of the Earth's orbit (1 AU) around the sun subtends at an angle of 1 arcsecond.

Why do we use absolute magnitude?

Because if two stars have the same apparent magnitudeit does not necessarily mean they emit the same amount of light or are the same size.

What is absolute magnitude?

The apparent magnitude a star would have if it was observed from a distance of 10 parsecs from earth.

What is the relationship between apparent and absolute magnitude and distance of a star from earth?

m - M =5 log (d/10)

m= apparent magnitude M= absolute magnitude d= distance of the star from earth (parsecs)

What is the distance modulus?

The difference between absolute and apparent magnitude. Distance modulus is negative for stars closer than 10 PC and positive for stars further away than 10 PC.

What is an ideal black body radiator?

one that absorbs and emits all wavelengths.

What does Wien’s displacement law state?

The black body radiation curve for different temperatures peaks at a wavelength that is inversely proportional to the temperature.

How do you convert from degrees to kelvin?

kelvin= Celsius + 273.15

What are the three types of light spectra?

continuous emission spectra, emission line spectra, absorption line spectra.

What is a continuous spectrum?

Created when photons of all wavelengths are emitted, a broad range of colours, produced by hot dense sources.

What is an emission spectrum?

Created when photons are emitted by excited electrons in hot gas, coloured lines on black background, produced by hot, low-pressure gases.

What is an absorption spectrum?

Created when phtons are absorbed by electrons in a cool gas, dark lines on a continuous spectrum, produced by light passing through cool, low-pressure gases.

How are stellar spectral lines created?

Photons produced by fusion reactions in a stars core move towards layers of gas in the outer atmosphere of the star, photons produced in core form a continuous spectrum, photons are absorbed by gas atoms which excite and re-emit other photons.

What is the balmer series?

a set of spectral lines primarily in the visible light range, that converge on the second energy level n=2, from the spectrum of hydrogen.

What are the spectral classes used today?

What is the intrinsuc colour of a star related to?

It’s peak wavelength which is attributed to its temperature.

How does the prominence of balmer lines vary across spectral classes?

0= weak,stars atmosphere too hot

B= slightly stronger, hydrogen likely to be ionised

A= strongest, high abundance of hydrogen in n=2 state

F= weak, stars atmosphere too cool

GKM= very weak/none, too little atomic hydrogen, far too cool

What are the conditions for nuclear fusion?

Very high temperature, pressure and density, so both nuclei have sufficiently high kinetic energy. Four hydrogen nuclei are fused into one helium nucleus, producing two gamma ray photons, two neutrinos and two positrons, massive amount of energy released.

What is radiation pressure?

the result of the momentum of the gamma ray photons after nuclear fusion.

What happens when the core temperature of a star reaches millions of degrees kelvin?

The fusion of hydrogen nuclei to helium nuclei begins, the protostars gravitational field continues to attract more gas and dust increasing the temperature and pressure of the core, with more frequent collisions, ke of particles increases, increasing probability of fusion, eventually when core becomes hot enough and fusion reactions can occur, they will begin to produce outward radiation pressure which balances the inward pull of gravity.

What happens when the star reaches a stable state?

the inward and outward forces are in equilibrium. As the temperature of the star increases and its volume decreases due to gravitational collapse, the gas pressure increases. The gas pressure and the radiation pressure act outwards to balance the gravitational force acting inwards. Temp increases the outward pressure increases, if outward pressure> gravitational force star will expand. if temp decreases, outward pressure decreases, if outward pressure< gravitational force the star will contract. As long as these two forces are balanced the star will remain stable.

What are the first three stages of the life cycle of stars?

Nebula: giant cloud of hydrogen and dust, gravitational attraction between individual atoms forms denser clumps of matter, inward movement of matter is called gravitational collapse

Protostars: gravitational collapse causes gas to heat up and glow, work done on particles of gas and dust by collisions causes increase in ke and thus temp. Eventually reaches millions of kelvin and nuclear fusion occurs

Main sequence star: star reaches stable state, temp increases volume decreases and gas pressure increases. fusion reaction begin in star core.

What forces act on a main sequence star?

Inward and outwards forces are balanced, radiation pressure and gas pressure exert outward forces. weight of gas exerts inward force.

What is a low mass and high mass star?

Low mass: star with mass less than 8 times the suns

high mass: star with mass more than 8 times the suns.

What are the next stages for a low mass star after main sequence?

Red giant: hydrogen fuelling star begins to run out, nuclear fusion slows, energy released by fusion reactions decreases. star initially shrinks and compresses core until fusion can continue in shell around core. Once fusion reactions start again, outer layers cool and expand. red giant is a large, low-temperature, luminous star in which helium nuclei are fused into more massive nuclei such as carbon, oxygen and berryllium.

Planetary nebula: outer layers of star are released, core helium burning releases massive amount of energy in fusion reactions.

white dwarf: solid core collapses under its own mass leaving remnant of core, extremely dense, hot star powered by the gpe it releases as it contracts.

What are the next stages for high mass stars after main sequence?

red super giant: follows the same process as the formation of a red giant, shell burning and core burning cycle in massive stars goes beyond that of low mass stars fusing elements up to iron.

supernova: the iron core collapses, the outer shell is blown out

Neutron star: the collapsed neutron core can remain intact, if neutron core mass is 3 times the solar mass, the pressure on the core becomes so great it collapses and forms a black hole.

What is a supernova?

An object which exhibits a rapid and enormous increase in absolute magnitude.

When are supernovae found to occur?

a type II - a supergiant collapses and then explodes

a type 1a - a white dwarf accrues matter and explodes.

What is a gamma ray burst?

A short, extremely high energy burst of gamma radiation emitted by a collapsing supergiant star. They can produce in a very short time period a huge amount of energy that is usually highly focused as narrow beams which are ejected from the poles of the exploding star.

What is a standard candle?

An astronomical objects of known brightness that can be used to calculate galactic distances.

What are the most common examples of standard candles?

cepheid variable stars, type 1a supernovae

How do type 1a supernovae occur?

the white dwarf increases in mass as it attracts material from its binary pair, eventually reaching a critical mass. the critical mass means the explosion is the same each time hence it produces a very consistent light curve.

What are the advantages of using a type 1a sypernova as a standard candle?

they reach the same peak of absolute magnitude each time, they are extremely bright.

What is a neutron star?

An extremely dense collapsed star made up of neutrons. extremely small and dense. the immense gravitational forces acting on the core crush electrons and protons until they combine into neutrons via reverse beta decay.

What is a pulsar?

A fast rotating neutron star that emits bursts of highly directional em radiation.

What is a black hole?

an object so dense that its escape velocity is greater than the speed of light, the boundary at which light and matter can not escape the gravitational pull of the black hole is called the event horizon.

What is Schwarzschild radius?

The radius of a black holes event horizon,

Rs= 2GM/ c²

where G= gravitational constant, M= mass of black hole, c= speed of light

What is sagittarius A*?

A supermassive black hole at our galactic centre.

What does the Hertzsprung- Russel diagram show?

it depicts luminosity of stars against their temps, stars clustered in distinct areas, most stars clustered in main sequence band where luminosity increases with temp, smaller clusters above are red giants and red supergiants that show increase in luminosity at cooler temps. below and to the left are white dwarfs, very hot but not very luminous. the diagram only shows stars in stable phases.

How can the life cycle of the sun be mapper onto a H-R diagram?