Astrophysics Flashcards
What is in the solar system?
Eight major planets orbit the sun in the same direction, the Earth’s orbit is approximately circular.
How are stars organized within the Milky Way galaxy? [3]
- Stars form stellar clusters, large groupings of stars gravitationally attracted to each other formed from the collapse of a single nebula
- Globular clusters have large numbers of mainly old, evolved stars
- Open clusters have smaller numbers of younger stars further apart
What does it look like when you zoom out of the galaxy? [3]
- Galaxies form “clusters” - the Milky Way is in the Local Group along with the Andromeda galaxy
- Clusters of galaxies form superclusters
- On a very large scale, the universe looks almost uniform.
What are binary stars?
Two stars orbiting a common centre
What is a Cepheid variable?
A star of variable luminosity whose luminosity has a well-defined period. The period is related to the absolute luminosity, and so can be used to estimate the distance to the star.
What is a comet?
A small body of mainly ice and dust orbiting the Sun in an elliptical orbit.
What is a constellation?
A group of stars in a recognizable pattern that appear to be near to each other in space.
What is the main sequence?
It is a star undergoing nuclear fusion of hydrogen into helium (eg. the sun). Found in the middle of the HR diagram.
What is a nebula?
Clouds of dust: carbon, oxygen, silicon, metals, hydrogen in the space between stars.
What is a neutron star?
The product of the explosion of a red supergiant. Very small and very dense - made almost entirely of neutrons.
How do stars carry out nuclear fusion? [3]
- The high temperatures inside the core allows the protons to overcome electrostatic repulsion, so nuclei can get close enough to fuse.
- High pressure means that nuclear are close enough to to give high probability of collision and fusion.
- Net result: 4 hydrogen nuclei turn into 1 helium nucleus. Energy is carried away by the photons and neutrinos produced in the reactions.
What are the two main forces inside a star?
- Gravitation tends to collapse the star inwards
- Radiation pressure opposes the gravitational pressure, keeping the star in equilibrium.
What are the following astronomical distances? How can you convert between them?
- A light year
- An astronomical unit
- A parsec
- The distance light travels in 1 year (ly)
- The average radius of Earth’s orbit (AU)
- FINISH
How can stellar parallax be used to measure astronomical distances? [3]
- Two measurements of the angular position of a star are made six months apart.
- The star appears to be displaced relative to the fixed background of distant stars.
- d (parsecs) = 1/p (in arc seconds)
What is the parallax angle?
(p) is the angle, at the position of a star, that is subtended by a distance equal to the radius of the Earths orbit around the sun.
What are the limitations of the parallax method?
If a star is too far away, the parallax angle is too small to be measured.
On Earth, distortions caused by the atmosphere also limit measurements.
What is the surface area of a sphere?
A = 4πR² for a sphere of radius R
What is luminosity? How can it be calculated?
The power radiated by a star, measured in watts. L = σAT^4 T = surface temperature A = surface area σ = Stephan-Boltzmann const
What is apparent brightness? How can it be calculated?
The received power per unit area (Wm^-2)
b = L/4πd²
Where d is the distance to the star.
What are stellar spectra?
Stars are assumed to radiate like a black body, over an infinite range of wavelengths.
What is Wien’s displacement law?
- The peak wavelength, λ₀ is related to surface temperature T by:
λ₀T = constant = 2.9 x 10^-3
The higher the temperature of a star, the shorter the peak wavelength of radiation will be.
What do absorption spectra show about a star? CHECK IN TEXTBOOK SAYS COOLER STARS WILL NOT RESULT IN MORE DARK BARS
- Dark bars represent the absorption of light of a specific wavelength by a specific chemical in the star’s atmosphere.
- The hydrogen in hotter stars is ionized, so atoms cannot absorb any light passing through them. Cooler stars have more electrons and can absorb more wavelengths
What does the Hertzsprung-Russell diagram illustrate? Where can different types of star be found? [6]
- The correlation between surface temperature (hot to cold on the x axis) and luminosity (very bright at the top)
- The main sequence shows stars that fuse hydrogen into helium.
- The sun is right in the middle of the main sequence
- Red supergiants are above red supergiants in the top right
- White dwarfs are in the bottom left.
- Red dwarves and blue giants occupy the ends of the main sequence.
What are some of the characteristics of red giants and supergiants?
- Large, cool stars with reddish appearance, tenuous (not very dense)
What are some of the characteristics of white dwarfs?
- dim, small, hot, whitish and dense. They are the naked core of red giants.
How is the mass of a star related to its luminosity?
L ∝ M^3.5
- Only applies to main sequence stars
What causes Cepheid variables to fluctuate in luminosity?
- There is a periodic expansion and contraction in the outer layers of the star, caused by radiation ionizing helium, it heating up and expanding, then cooling, contracting, and helium nuclei recombining with electrons.