10. Space Flashcards
Define luminosity
What symbol and units are used?
The rate of light energy released or power output of a star.
Symbol: L
Units: W or Js⁻¹
Define Intesity
What symbol and units are used?
The power received from a star (its luminosity) per unit area.
Symbol: I
Units: Wm⁻²
What about a star follows the inverse square law?
It’s intensity.
Showed by I=L/4πd².
(4πd² beacuse light from a star is assumed be emitted equally in all directions from a point.)
What is parallax?
The apparent change of position of a nearer star in comparison to distant stars in the background
Why does parallax happen?
Because the orbit of the Earth around the Sun.
How is parallax measured
Angle of parallax (θ).
by measuring the angle to a star and seeing how this angle changes as the Earth changes position.
What can you deduce from the angle of parallax about a star’s distance?
The greater the angle of parallax, the closer the star is to the Earth.
What is meant by Astronomical Unit (AU)?
The average distance between the centre of the Earth and the centre of the Sun.
What is meant by Parsec (pc)?
The distance at which the angle of parallax is 1 arcsecond (1/3600th of a degree).
What is meant by Light Year (ly)?
The distance that an EM wave travels in a year in a vacuum.
What is a standard candle?
An object of known luminosity.
How are standard candles used to measure distances?
By measuring the intensity detected from the light source on Earth and using the inverse square law equation to calculate its distance away.
What does a Hertzsprung-Russell (HR) diagram show?
The stellar luminosity of a star against its temperature
What can you infer from a Hertzsprung-Russell (HR) diagram?
What spectral class that star belongs to.
Where are Red Giants found on a Hertzsprung-Russell diagram?
Top Left
with red supergiants being at the very top
Where are Blue Giants found on a Hertzsprung-Russell diagram?
Top left.
Where are Main Sequence Stars found on a Hertzsprung-Russell diagram?
From top left to bottom right in a y=-x³ shape.
Where are White dwarfs stars found on a Hertzsprung-Russell diagram?
Bottom left.
What direction do the scales go on a Hertzsprung-Russell diagram?
X-axis: Temp Decreasing
Y-axis: Luminosity Increasing.
Both exponential.
What main feature dictates the lifecycle of a star?
Its mass.
How are protostars formed?
- Clouds of gas and dust (nebulae) have fragments of varying masses that clump together due to gravity.
- Irregular clumps rotate gravity/conservation of angular momentum spins them inwards.
- This forms a denser centre – a protostar.
What happens to form a main-sequence star?
- High temps and pressures allow for fusion.
- The inward force of gravity and the outward force due to fusion are in equilibrium – the star is stable.
- Hydrogen nuclei are fused into helium.
- The greater the mass of the star, the shorter it’s main sequence period because it uses it fuses at a faster rate.
What happens to form a Red Giant star?
- Once the hydrogen runs out, the temperature of the core increases.
- It begins fusing helium nuclei into heavier elements (E.g. Carbon, Oxygen and Beryllium).
- The outer layers of the star expand and cool.
What happens to form a White Dwarf star?
- When a red giant has used up all its fuel, fusion stops and the core contracts as gravity is now greater than the outward force.
- The core becomes very dense.
- A white dwarf will eventually cool to a black dwarf.
What happens to form a Red Supergiant star?
- A high-mass star runs out of hydrogen nuclei.
The same process for a red giant occurs but on a larger scale.
What happens to form a Supernova?
- When all fuel runs out, fusion stops and the core collapses inwards very suddenly and becomes rigid (as the matter can no longer be forced any closer together).
- The outer layers of the star fall inwards and rebound off of the core, launching them out into space in a shockwave.
- As the shockwave passes through surrounding material, elements heavier than iron are fused and flung out into space.
- The remaining core depends on the mass of the star.
What happens to form a Neutron Star?
- The core of a large star collapses, gravity is so strong that it forces protons and electrons together to form neutrons and neutrinos (which are flung out in the supernova).
What happens to form a Black Hole?
- When the core of a giant star collapses, the neutrons are unable to withstand gravity forcing them together.
- The gravitational pull of a black hole is so strong that not even light can escape.
What is the Doppler Effect?
The compression or spreading out of waves that are emitted or reflected by a moving source.
What effect does the Doppler Effect have on line spectra?
Causes the line spectra of distant objects to be shifted either:
- Towards the blue end of the visible spectrum when they move towards the Earth (blue-shift)
OR
- Towards the red end of the spectrum when they move away from the Earth (red-shift).
Why is redshift evidence fro the expanding universe?
- Distant objects are red-shifted.
- The more distant the object, the greater its red-shift.
When using the redshift formula what is the exception for its use?
It can only be used when v is much smaller than c.
What is Hubble’s law used for?
For objects at cosmological distances.
In Hubble’s law, what are the units of distance?
Megaparsecs.
How can we suggest the universe began from one point.
- The redshift of distant objects shows that they are moving away from us.
- This suggests that the universe is expanding.
- It would be reasonable to assume that the universe began from one point
- This point is a singularity that was infinitely small and infinitely hot.
How can we estimate the age of the universe?
Using Hubble’s Law:
v = H₀d v/d = H₀
1/H₀ = d/v = time
∴ t = 1/H₀
If we can’t detect dark matter, how do we know it exists?
- By considering the centripetal force exerted on stars in the outer orbits of a galaxy we expect them to travel slower than stars closer to the galactic centre.
- As centripetal force is inversely proportional to the distance from the centre.
- But it has been observed that all stars in a galaxy tend to travel at the same speed regardless of distance from the centre of the galaxy.
- This suggests that the stars have a larger mass than they appear.
- This extra mass is believed to be caused by dark matter.
