AST201 midterm 1

What is the overall composition of the universe?

Mostly dark energy, dark matter, and less than 5% ordinary matter.

What is one piece of evidence for the existence of dark matter?

Galaxies in clusters move at faster speeds than explained by their gravity, indicating 'missing mass'.

What is velocity?

Directional speed (objective).

What is acceleration?

Something that is moving fast and getting faster (subjective speed).

What does it mean for the speed of light to be 'invariant'?

It stays the same for everyone.

What is time dilation?

Moving clocks run slow.

What is the Twin Paradox?

Each twin sees the other's clock running slower; the outgoing twin ages less.

What are the four fundamental forces of nature?

Strong nuclear force, electromagnetism, weak nuclear force, and gravity.

What role does the strong nuclear force play in the universe?

It holds atomic nuclei together.

How does gravity affect masses?

It makes masses attract one another.

What is Newtonian gravity?

Gravity as a classical force.

What is Einstein's general relativity?

Gravity as a consequence of spacetime curvature.

What is the Equivalence Principle?

You can't tell the difference between a gravitational force and an external acceleration.

What is escape speed?

The speed required for a projectile to break free from Earth's gravity.

How does escape speed change with distance?

It decreases as you move further away from the mass.

What is the Schwarzschild radius?

The distance from the singularity to the event horizon, depending only on the black hole's mass.

What is the event horizon of a black hole?

The outer boundary where the escape speed equals the speed of light.

Why are black holes not considered 'cosmic vacuum cleaners'?

They only have as much gravity as whatever they formed from.

What is one observational evidence for black holes?

X-ray binary systems showing strong x-rays from a companion star with no visible star.

What is the Event Horizon Telescope?

It provides direct imaging of a black hole with its accretion disk.

What do gravitational wave measurements indicate?

They show that every object with mass distorts spacetime.

What happens when two massive objects orbit one another?

Their continuous change in spacetime curvature goes outward in a gravitational wave.

What can correlating gravitational wave observations detect?

Distant black hole mergers.

What is located at the center of the Milky Way galaxy?

A supermassive black hole known as Sagittarius A*.

How can the mass of Sagittarius A* be calculated?

By observing the motion of stars around it.

What evidence supports the existence of a supermassive black hole?

Extremely strong evidence from the motion of stars near the center of our galaxy.

What is gravitational lensing?

It bends light around massive objects.

What would happen to Earth if the Sun was replaced by a 1 solar mass black hole?

Earth would continue to orbit as usual but would become very cold.

What is the mass range for stellar black holes?

5 to 10 solar masses.

How do intermediate black holes form?

From binary black holes that fuse together.

What is the mass range for supermassive black holes?

Millions to billions of solar masses.

What is the radius of the Sun compared to Earth?

Approximately 100 times the radius of Earth.

What is the distance from the Earth to the Sun in light minutes?

8 light minutes (1 AU).

What does a blackbody spectrum produce?

A continuous spectrum.

What are absorption lines in the solar spectrum?

They show where electrons are absorbing or emitting light.

What is the nucleus of an atom composed of?

Protons and neutrons.

What happens when an electron drops from a high to low energy shell?

It emits light.

What is the unique set of electron shells for each chemical element used for?

To emit or absorb a unique set of wavelengths of light.

What determines the temperature of a celestial object?

The wavelength at which its blackbody spectrum peaks.

Why don't we see green stars?

Equal amounts of red, green, and blue light combine to appear white.

What is nuclear fusion?

The fusion of 4 protons into a helium nucleus.

What are the main inputs of the proton-proton chain?

4 Hydrogen atoms.

Why can neutrinos escape the Sun easily?

They barely interact with other matter.

What is luminosity?

How bright a light source is intrinsically.

What is apparent brightness?

How bright a light source appears to a given observer.

What is a light year?

The distance light travels in one year.

How does the inverse square law of light relate to a star's luminosity?

The amount of light received from a star falls off with the square of the distance from the star.

What is the sequence of stellar spectral types?

O B A F G K M.

What distinguishes stars of different spectral types?

Their surface temperatures.

What do O-class stars represent on the H-R diagram?

They are large and blue.

What do M-class stars represent on the H-R diagram?

They are small and red.

What happens to the mass and lifetime of stars from M to O class?

Mass increases and lifetime decreases.