Astronomy pt2

What is a black body?

An ideal object that perfectly absorbs all incident radiation and emits it all again isotropically.

What is the Wien tail in a blackbody spectra?

At short wavelengths, the spectrum cuts off sharply as a result of the exponential in the denominator of Planck’s Law.

What is the Rayleigh-Jeans tail in a blackbody spectra?

As λ → ∞ (long wavelengths) the spectrum declines more slowly.

What is Kepler’s 3rd Law?

P² ~ R³

Talk to me about CMB

The CMB is relic radiation from the early Universe, released when the Universe first became transparent about 380,000 years after the Big Bang

Before this, photons were constantly scattered by free electrons, so the Universe was opaque.

As the Universe expanded and cooled to about 3000K electrons and protons combined to form neutral hydrogen. This is called recombination. After this, photons decoupled from matter and travelled freely through space.

The radiation originally had a blackbody spectrum at about 3000K, but expansion stretched its wavelength by a factor of about (1100). So today it is observed as microwave radiation with temperature 2.7K.

The CMB is nearly uniform, but it has tiny spatial temperature fluctuations called anisotropies.

These fluctuations show that there were slight over-dense regions in the very early Universe.

Those over-dense regions acted as the seeds of structure formation. Over time, gravity caused them to grow into the galaxies, galaxy clusters, and large-scale structure we see today.

A good exam sentence would be:

CMB anisotropies show early over-densities, which later grew by gravity into galaxies and large-scale structures.

How can a ground-based telescope operate near its diffraction limit even when atmospheric seeing would normally blur the image?

It can use adaptive optics to operate near the diffraction limit. Adaptive optics uses deformable mirrors that adjust in real time to correct distortions in the wavefronts of light caused by the Earth’s turbulent atmosphere.

What happens to objects with M<0.08M⊙

They become brown dwarfs. They are not true stars because they never get hot enough to fuse hydrogen.

What happens to stars with 0.08M⊙<M<0.8M⊙?

They are low mass stars with luminosity so low that they stay on the main sequence longer than the age of the current universe. They will eventually evolve to white dwarfs but we will never see this happen.

What happens to stars with 0.8M⊙<M<5–8M⊙?

They evolve into white dwarfs.

What happens to stars with 5–8M⊙<M<25–40M⊙?

Neutron stars

What happens to stars with M>25–40M⊙?

Black holes