A variable star is one whose apparent brightness changes as viewed from earth. Cepheid variables are a certain type of variable star whose apparent brightness changes in regular cycles lasting from 3 to 50 days. Astronomers use them to measure distances in space.

Q: 4 describe and explain the characteristic of Hertzsprung-Russell diagram.

what is the significant and use of this diagram? how is a black hole formed? what are the properties of black holes? compute the Schwarzschild radius for the sun.

What happens when the star shrinks to the radii? what is the most likely place to find a black hole?

The Hertzsprung-Russell diagram is a scatter-graph of stars that allows astronomers to plot their absolute luminosity against their specific classifications and temperatures. This technique demonstrated that the temperature and luminosity of stars are not random but related in such a way as to permit classification.

Stars much larger than that explode much more violently and completely than those that end up as dwarf stars. They produce black holes whose only features are vestigial remains of the original star such as mass and electromagnetic signature. They are regions of infinite gravity from where even light cannot escape.

The Schwarzschild radius is the size that a given object would have to be for its matter to be...

Beyond that density, light is no longer visible from that (former) star. The sun would reach that point once it radius approached 3 km.
Black holes are most likely to be found at the centers of galaxies.

Q: 5-1) Describe the characteristics of our Milky Way Galaxy.

2) describe the dark matter, how is the dark matter is detected?

3) describe what is meant by dark energy.

The Milky Way Galaxy is a 13 billion-year-old barred spiral galaxy that is approximately 100,000 light years in diameter that rotates in space at a rate of one complete rotation every 15 to 50 million years. It contains approximately 400 billion stars.

Dark matter is believed to account for as much as 90% of the matter in the universe. Because dark matter does not emit any energy, it cannot be detected directly. Its existence is inferred from the gravitational effects it seems to have on the matter around it, especially in connection with galaxies and their motion. Dark matter is believed to consist of heavy particles that interact with the physical world only through the weak nuclear force.

Dark energy is believed to be the reason that the universe continually expands. It is hypothesized that dark energy accounts for approximately three-quarters of the mass-energy of universe.