Classification of Stars (AQA A Level Physics): Flashcards

• I = brightness, or observed intensity on Earth (W m⁻²)

• L = luminosity of the source (W)

• d = distance between the star and Earth (m)

The inverse square law of radiation assumes the source can be treated as a point, that power radiates uniformly through space, and that no radiation is absorbed or scattered between the star and Earth.

False.

Intensity follows an inverse square law, so doubling the distance reduces the intensity to one quarter of its original value.

The closer star has the greater apparent brightness.

The astronomical unit is the mean distance from the centre of the Earth to the centre of the Sun.

A light-year is the distance travelled by light in one year.

A parsec is the distance at which the radius of the Earth's orbit (1 AU) around the Sun subtends an angle of 1 arcsecond.

• 1 AU ≈ 1.5 × 10¹¹ m

• 1 light-year ≈ 9.5 × 10¹⁵ m

• 1 parsec ≈ 3.1 × 10¹⁶ m

One arcsecond is equal to 1/3600 of a degree.

• Solar system: astronomical unit (AU)

• Interstellar distances: light-year (ly) and parsec (pc)

False.

The parsec (≈ 3.1 × 10¹⁶ m) is far larger than the astronomical unit (≈ 1.5 × 10¹¹ m).

Absolute magnitude is the apparent magnitude a star would have if it were observed from a distance of 10 parsecs from Earth.

• m = apparent magnitude

• M = absolute magnitude

• d = distance from Earth (pc)

The distance modulus is the difference between a star's apparent and absolute magnitude (m − M), used to quickly determine its relative distance.

• Closer than 10 pc: negative

• Further than 10 pc: positive

Values of absolute magnitude are generally more negative than their associated apparent magnitude, since most stars are further than 10 parsecs away.

Absolute magnitude compares stars as if observed from the same distance (10 pc), so it reflects intrinsic brightness rather than being affected by how far away each star actually is.

False.

Stars with the same apparent magnitude can have different absolute magnitudes, since apparent magnitude also depends on distance from Earth.

A black-body radiator is an object that absorbs and emits all wavelengths of radiation. Stars are the best real-world approximation of a perfect black body.

The peak moves to a lower wavelength and a higher intensity.

Wien's displacement law states that the peak wavelength of a black-body radiation curve is inversely proportional to its absolute temperature.

A blue star has a surface temperature above 33 000 K.

False.

Blue stars are the hottest (>33 000 K), while red stars are the coolest (<3500 K).

Intensity (y-axis) against wavelength (x-axis).

Stefan's law (the Stefan-Boltzmann law) states that the total energy emitted by a black body per unit area per second is proportional to the fourth power of its absolute temperature.

• Its absolute temperature

• Its surface area

• P = total power emitted across all wavelengths (W)

• σ = the Stefan-Boltzmann constant

• A = surface area of the body (m²)

• T = absolute temperature of the body (K)

The surface area of a star of radius r is given by A = 4πr².

False.

Since , doubling the temperature increases the luminosity by a factor of sixteen, not two.

A continuous spectrum is created when photons of all wavelengths are emitted. It is produced by hot, dense sources, such as the cores of stars, and appears as a broad, uninterrupted range of colours.

An emission spectrum is created when photons are emitted by excited electrons in a hot, low-pressure gas, such as a nebula. It appears as discrete coloured lines on a black background.

An absorption spectrum is created when photons are absorbed by electrons in a cool, low-pressure gas, such as a star's photosphere. It appears as dark lines at discrete wavelengths on a continuous spectrum.

Each element produces a unique pattern of spectral lines. If an element is present in a star, its characteristic pattern appears as dark lines in the star's absorption spectrum, acting like a fingerprint.

The Balmer series, which involves electron transitions to or from the second energy level (). Balmer series photons lie in the visible spectrum.

Hot, dense sources, such as the cores of stars, produce continuous spectra.

False.

The absorption lines correspond to the same wavelengths as the emission lines of that element, since both arise from the same electron energy-level transitions.

A spectral class is a system for classifying stars based on their intrinsic colour, temperature and prominent absorption lines. The system of seven classes was devised by the astronomer Annie Jump-Cannon.

• O

• B

• A

• F

• G

• K

• M

5000–6000 K; yellow-white.

Class A. Its atmosphere is hot enough to give a high abundance of hydrogen atoms with electrons in the state, without ionising most of the hydrogen.

Hydrogen and helium are in much higher abundance in the atmospheres of the hottest stars, so their spectral lines dominate.

At low temperatures, there may not be enough energy to excite atoms or break molecular bonds, resulting in TiO and neutral atoms being seen in classes K and M.

False.

Class O atmospheres are so hot that hydrogen is largely ionised, so Balmer lines are weak. Class A stars have the strongest Balmer lines.

Radiation pressure is the outward-acting pressure resulting from the momentum of gamma-ray photons produced by fusion reactions in a star's core.

• A very high temperature (on the scale of 100 million kelvin)

• Very high pressure and density

• One helium nucleus

• Two gamma-ray photons

• Two neutrinos

• Two positrons

Radiation pressure and gas pressure.

If the temperature of a star increases, the outward pressure will also increase; if outward pressure exceeds gravitational force, the star will expand.

False.

An increase in temperature increases the outward pressure. If this exceeds the gravitational force, the star will expand, not contract.

A protostar is a hot, glowing ball of gas and dust formed by the gravitational collapse of a nebula, before nuclear fusion begins. Protostars can be detected by telescopes observing infrared radiation.

A main sequence star is one in which radiation pressure is produced by the thermonuclear fusion of hydrogen nuclei into helium nuclei, with inward and outward forces in equilibrium.

A red giant is a large, low-temperature, luminous star in which helium nuclei are fused into more massive nuclei such as beryllium, carbon and oxygen.

A white dwarf is an extremely dense, hot star powered by the gravitational potential energy released as it contracts, rather than by nuclear fusion.

About eight times the mass of the Sun.

The carbon-oxygen core is not hot enough for further fusion, so it collapses, and the outer layers of the star are released, forming a planetary nebula.

Protostars can be detected by telescopes that observe infrared radiation.

False.

The Sun is a low-mass star, so it will end its life as a white dwarf, not a neutron star.

A red supergiant forms via the same process as a red giant, but the shell-burning and core-burning cycle continues further, fusing elements up to iron.

Greater than eight times the mass of the Sun.

The iron core collapses, and the outer shell is blown out in an explosive supernova.

If the remaining neutron core has a mass greater than three times the solar mass, it collapses further into a black hole. Otherwise, it remains a neutron star.

The shell-burning and core-burning cycle in massive stars fuses elements up to iron.

False.

A neutron core only collapses into a black hole if its mass is greater than three times the solar mass; otherwise it remains a neutron star.

A supernova is an object which exhibits a rapid and enormous increase in absolute magnitude.

• Type II: a supergiant star collapses and then explodes

• Type 1a: a white dwarf accrues matter and explodes

A gamma-ray burst is a short, extremely high-energy burst of gamma radiation emitted by a collapsing supergiant star.

Between 10⁴⁴ and 10⁴⁷ J — comparable to the total energy output of the Sun over its ten-billion-year lifespan.

It is highly focused into narrow beams, which are ejected from the poles of the exploding star.

So far, gamma-ray bursts have only ever been detected at great distances, and never in the Milky Way.

False.

Gamma-ray bursts have only ever been detected at great distances, and never in the Milky Way.

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

• Cepheid variable stars

• Type 1a supernovae

The Chandrasekhar Limit is the critical mass at which an accreting white dwarf explodes as a supernova. Because this mass is always the same, the explosion is the same each time, producing a consistent light curve and peak absolute magnitude.

A type 1a supernova occurs when a white dwarf in a binary system accretes mass from its companion until it reaches the Chandrasekhar Limit and explodes.

Type 1a supernovae are extremely bright, so they can still be observed and used to calculate distances to the furthest galaxies.

False.

Every type 1a supernova explodes once the white dwarf reaches the same critical mass (the Chandrasekhar Limit), so they always reach the same peak absolute magnitude.

An extremely dense, collapsed star made up of neutrons.

• A core mass between 1.4 and 3 solar masses forms a neutron star

• A core mass greater than 3 solar masses forms a black hole

Further collapse of a neutron star is prevented by neutron degeneracy pressure.

A rapidly rotating neutron star that emits periodic bursts of highly directional electromagnetic radiation, such as radio waves.

Pulsars emit radiation periodically (often radio waves, sometimes X-rays and gamma rays), which makes them easier to identify.

The boundary around a black hole beyond which the escape velocity exceeds the speed of light, so nothing, not even light, can escape.

The supermassive black hole at the centre of the Milky Way, called Sagittarius A\*, has a mass of about four million solar masses.

False.

Black holes cannot be seen directly, since photons cannot escape beyond the event horizon; they are instead detected from their gravitational effects on nearby stars.

The diagonal band on a Hertzsprung–Russell diagram where most stars are found; for these stars, luminosity increases with surface temperature.

• Luminosity on the y-axis (dim at the bottom, bright at the top)

• Temperature on the x-axis (hot on the left, cool on the right)

They are much larger than main sequence stars, so their large surface area makes them more luminous despite being cooler.

A protostar collapses from a cold cloud of gas; it is initially a dim, cool star, then moves onto a fixed position on the main sequence determined by its mass.

The Hertzsprung-Russell diagram only shows stars in stable phases, since transitory phases such as supernovae happen quickly relative to a star's lifetime.

They are hot but not very luminous, so they must be much smaller than main sequence stars.

When the supply of helium runs out, nuclear fusion stops and the star collapses into a white dwarf; its surface temperature becomes very hot (~10 000 K), but its small surface area gives it a low luminosity.

False.

The brightest, most massive stars have the shortest lifetimes, as they use up their nuclear fuel at a much faster rate than dimmer stars.

Black holes emit no light, so they have no luminosity or temperature that can be plotted.