Astrophysics cosmology Test and Flashcards

What is the doppler effect?

The change in wavelength and frequency of a wave as the source moves away or towards the observer. As the source moves towards the observer, the waves are compressed and the wavelength decreases (blue shift). As the spurce moves away from the observer, the waves spread out and the wavelength increases (red shift).

What is red shift?

Red shift is the shift in wavelength and frequency of waves from a retreating source towards/beyond the red end of the electromagnetic spectrum. Cosmologcal red shift is evidence for the bigbang.

Formula for red shift

Can only be used when v is much smalle than c

State Hubble's law

The velocity of receeding galaxies is proportional to their distance from Earth

How is Hubbles' law used to estimate the age of the universe?

Simplified light ccurve for a binary system with explainations

What is the big bang theory?

13.8 billion years ago scientists believe the universe exploded from an extremely hot and dense point and is still explanding now

What is the evidence that led to the acceptance of the big bang theory?

CMBR (cosomological microwave background radiation) is the heat sinature left behind from the big bang. The EM radiation released in the explosion shifted from extremely high energy waves into the microwave region as the universe expanded,stretching out the waves. CMBR has a black body distrubution with a peak that corresponds to a temperature of 2.7K. There was nuclear fusion of hydrogen into heilium which explains the large abundance in today's universe.

How is the relative abundance of hydrogen and helium used as evidence for the Big Bang?

During the Big Bang, Hydrogen was converted into Helium via nuclear fusion. The expansion of the universe caused it to cool meaning it was no longer hot enough to fuse elements. ~ 1/4 of all hydrogen was converted into Helium, resulting in a ratio of H:He of 3:1 The observed distribution of matter now is 73% hydrogen 25% helium and 2% everything else which follows the predicted ratio

What are exoplanets?

Exoplanets are planets that are not in our solar system

Why is observing expolanets difficult?

Direct observation of exoplanets are diffuicult as their light is often obscured by the stars they orbit They tend to be too close together for the telescope to observe them

How to detect exoplanets?

Radial velocity method Transit method

Transit method

As a planet moves between the star it orbits and the Earth, the star's brightness appears to decrease slightly. This can be detected and used to calulate the diameter of the planet There is a low chance of this orbit being in the right place to measure this so it is mainly useful for detecting planets with small orbits As they are more likely to cross the star's disk

Radial velocity method

A star and a planet will orbit their common centre of mass, this means the star will have tiny variations in its distance from Earth shown by red and blue shifts in it spectrum

Light curve for an exoplanet transiting a star

What are quasars?

Nucleus of an active galaxy ; a supermassive black hole surrounded by discs of matter. As matter falls into the holw jets of radiation are emitted from the poles of the quasar.

What suggests that quasars are extremely distant objects?

Large optical red shift shows quasars are the most distant observable objects, from the inverse square law we know they are extremely powerful, with the same energy output as several galaxies. They were intitially found to be powerful radio sources but with further telescope developments we now know they emit all wavelengths of EM radiation

inverse squaare law

As light emits from a source, it spreads out in all directions. Its intensity therefore decreases. Intensity at Earth = Intensity of a star / (distance from Earth)^2

The brightness/intensity ratio of two stars equation

m-apparent

Quasars

Quasars as the most distant measurable objects. Formation of quasars from active supermassive black holes.

Give an example from astronomy of a situation where the double Doppler effect would have to be applied, explaining why it is necessary to apply the double effect.

Rotation of a star

Double doppler effect because body acts as source for return signal

Explain a difficulty with using the transit method to detect a Eartg like planet orbiting a sunlike star

Transit method measures how much light is blocked by the planet

Planet is pequeno so little light is blocked

When the apparent magnitude is five can it be viewed from earth

Yes as still lower than the greatest value of six in the hipparcos scale

Why the quasar stopped emitting radiation

Quasars formed about black holes, black hole does not have matter falling into it

Rayleigh criterion

Rayleigh criterion identifies the minimum angle subtended between the two objects which can be resolved

The minimum angle is when the central maximum of the diffraction pattern of light from one object coincides with the first minimum of the diffraction pattern of the second object

Flashcards: Astrophysics cosmology