Can all stars collide one day?
Considering the number of stars that exist, star collision events are relatively rare. The distance between the stars tends to be large. If they are in a binary system, something must constantly cause them to lose energy for there to be a collision. This occurs for example in binary systems of neutron stars (which are not technically stars anymore), but there is 1 collision in the Milky Way every 3000 to 60000 years. Some other event may occur and cause a star to wander around the galaxy and it may encounter another star, but even when whole galaxies collide, it is very rare for one star to collide with another.
What happens when a star with M< 8 M_sun undergoes a planetary nebula event near another star? Do the gases hit it? Is she pulled closer to the white dwarf?
What happens in a system of two stars depends very much on the distance between these stars. There can be mass transfer between the stars over the evolution of the two stars, for example. When a star in a double system ejects its outer layers in the form of a planetary nebula, the gases can reach the companion star as it expands but the speed is relatively low. It would not be pulled close to the white dwarf because the white dwarf will ultimately be of lower mass than before it ejected its outer layers.
What is the connection between the lifetime of a star and its mass?
The lifetime of the star depends on its mass, the heavier the star, the shorter the lifetime, and can range from a few million to billions of years. An approximate relationship is that the lifetime is proportional to 1/mass2,3. But this depends on if the star evolves independently or if it interacts with another star in a double or triple system.
Why can't we see planets in our own system, but we can see thousands of stars, which are much further away?
The amount of light reaching an observer from a given source decreases with the square of the distance between the source and the observer. The planets only reflect the light that reaches them. In the solar system we can see with the naked eye the planets that are closest to the Sun, which receive a lot of light, and even Jupiter and Saturn, which, although farther away from the Sun, are quite large and reflect sunlight pretty well. For the more distant ones we need telescopes. Stars are huge (much larger than planets) and emit a lot of light. Even so, not all stars can be seen with the naked eye. This will depend on how much light the star emits (depending on its mass and the phase of life it is in) and how far it is from us.
Can it happen that all the stars die at once and there are no more stars in the sky?
Each star has its own lifespan, and they are created at different times, so they will not all die at once. The Milky Way is still forming stars, and it will take a long time for star formation to stop in the Galaxy, and for the lightest stars to all die (trillions of years). There will still be stars shining for a long, long time in the Milky Way.
Why do all stars have names?
Stars are given names to make it easier to identify and study them. The ancients used names like Sirius, Canopus, Rigel... As observation techniques improved and new stars started to be observed, there was a big increase in their number and they started to be identified by the name of the constellation where they are followed by a Greek letter that indicates their brightness (the brightest is alpha, followed by beta, and so on). But today we know too many stars to name them all with "creative" names. Then they get a number that represents their positions in the sky and are associated with the catalog that describes them. For example, Sanduleak -69th 202 was the star that gave rise to supernova 1987A.
How long does the star live and how long does it spend in each phase?
How long a star lives varies according to its mass. The heavier ones, with more than 60 solar masses, live for a few million years, and it is estimated that the lighter ones, with less than 20% of the mass of the Sun, can live for a few trillion years. Stars spend about 90% of their lives in the main sequence phase, in which they fuse hydrogen into helium inside. The rest of the time, they spend in the form of giants or supergiants.
Why do stars only appear in the middle of the night and we cannot observe them during the day?
In fact, we see one star during the day: the Sun. And that is why we have difficulty seeing the other stars, since the Sun is very bright, and its light is scattered by the atmosphere, obscuring the brightness of other objects. It is a similar effect to light pollution in large cities, which makes it more difficult to see less bright stars in these locations. During a total eclipse of the Sun, it is possible to observe the brightest stars as well.
What are the stars and the universe made of?
Stars are basically made of hydrogen and helium with an amount of heavier elements that depends on how old it is and the region in which it was formed. But this amounts to only a few percent of its mass. The Universe is more mysterious... A very small fraction, less than 5%, of what composes it is made of the same stuff as stars, planets, and what we see in our everyday lives. But there seems to be two other components called dark matter, which makes up about 27%, and dark energy, with the rest, almost 70%! We can only identify these two components by the effects seen in some events in the universe, and what exactly they are is one of the big open questions in astronomy and particle physics today.
Is it true that inside a black hole time does not pass? If not, what exactly happens inside it?
Nobody knows what happens inside the black hole because nothing escapes from the black hole, so we have no way to get information from inside it. There is a boundary, called the Schwarzschild radius, which delimits the region from which we can get information around the black hole. As something approaches this radius (which is not a physical boundary, only a mathematical one) time begins to pass more and more slowly until it would "freeze" over the Schwarzschild radius.