# The light of which star is next to reach the earth?

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I heard that the sky wouldn't be dark at night if the light of every star had already reached the earth.

1. Of which (currently invisible) star will the light reach the earth next and when will that be?
2. What was the last star that we can now see at night to reach the earth and when was that?
3. How can we know (1) if the light hasn't even reached us yet? Or can't we know and will a star suddenly appear without anyone being able to predict that?

I think you may be thinking of Olbers' paradox . This supposes that if the universe were infinite in time and extent, and stars were more or less randomly placed, then every line of sight would end at a star, and so the sky would be as bright as the surface of a star.

The paradox can be resolved by supposing that the universe is not infinite in time, but had a beginning.

We don't see stars suddenly appearing as their light reaches us. What we see is stars in all stages of their development. When we look the nebulae where stars are born we see clumps of gas in the process of collapsing to form stars. These are called protostars. Now the process of stellar formation takes a long time: about a million years for a medium sized star. The start of a star is not a sudden "turning on". Instead, as the core of the protostar shrinks and heats up, nuclear fusion starts; slowly at first, then increasing until it is able to prevent further collapse and heating.

Protostars usually are dim in visible light, they can be found with infra-red telescopes.

Since the process is slow and there isn't a sudden turning on, your questions cannot be directly answered. However, there is an object HOPS383 which is 1400 light years distant, which between 2004 and 2008 had a "growth spurt" and became visible in infra-red for the first time. It can't be seen in visible light yet - so in a sense this protostar answers your question 1 and 2, in different wavelengths.

To fully answer your question, you have to take into account the fact that light travels at the speed of light ($approx 300,000 mathrm{km/s}$). The further you look, the 'earlier' you look. Imagine that you are watching a star that is $3 mathrm{Glyr}$ away (the distance traveled by light in 3 billion years), then the light you see has been emitted 3 billion years ago.

To answer your question, you thus have to find the earliest star created in the Universe we can see. The part of the Universe we can see is restricted to the part located at $14 mathrm{Glyr}$ and less, because it is the longest distance light could have traveled since the beginning of the Universe.

So now the question is: when did the first star form? A recent paper made by astronomers reported that the first stars (which are also the farthest!) are to be found $800 mathrm{Myr}$ after the Big Bang, and they are the in CR7 galaxy. With increasing time, very far and very dim stars will appear in the sky, but you won't be able to see them with your naked eyes.

To be able to see a star appear in the sky with your naked eye, you probably have to think about supernovae. Supernovae are massive stars that explode at the end of their life, increasing dramatically their emitted light in a matter of hours. If the exploding star was too dim before exploding to be visible, it will look as if a bright star appeared in the sky once it has exploded. You can find a sunnary of the observation of these stars on wikipedia.

The story of humanity is a story of great leaps – out of Africa, across oceans, to the skies and into space. Since Apollo 11’s ‘moonshot’, we have been sending our machines ahead of us – to planets, comets, even interstellar space.

But with current rocket propulsion technology, it would take tens or hundreds of millennia to reach our neighboring star system, Alpha Centauri. The stars, it seems, have set strict bounds on human destiny. Until now.

In the last decade and a half, rapid technological advances have opened up the possibility of light-powered space travel at a significant fraction of light speed. This involves a ground-based light beamer pushing ultra-light nanocrafts – miniature space probes attached to lightsails – to speeds of up to 100 million miles an hour. Such a system would allow a flyby mission to reach Alpha Centauri in just over 20 years from launch, beaming home images of its recently-discovered planet Proxima b, and any other planets that may lie in the system, as well as collecting other scientific data such as analysis of magnetic fields.

Breakthrough Starshot aims to demonstrate proof of concept for ultra-fast light-driven nanocrafts, and lay the foundations for a first launch to Alpha Centauri within the next generation. Along the way, the project could generate important supplementary benefits to astronomy, including solar system exploration and detection of Earth-crossing asteroids.

A number of hard engineering challenges remain to be solved before these missions can become a reality. They are listed here, for consideration by experts and public alike, as part of the initiative’s commitment to full transparency and open access. The initiative will also establish a research grant program, and will make available other funding to support relevant scientific and engineering research and development.

## Distance to the Nearest Star

The Sun is about 93 million miles from the earth. The star nearest to the Sun is Proxima Centauri. Astronomers measure the distance between stars in units called light-years. A light-year equals 5.88 million million miles (9.46 million million kilometers). This is the distance light travels in one year at a speed of 186,282 miles per second (299,792 kilometers per second). Proxima Centauri is 4.3 light-years from the Sun. It is a dim red star in the constellation of Centaurus that lies at a distance of over 40 million million kilometers, some 270,000 times greater than the distance between the earth and the sun.

The nearest stellar neighbors to the Sun are three stars that make up a multiple system. To the naked eye the system appears as a single bright star, Alpha Centauri. Alpha Centauri is a double star -- two stars revolving about each other that are too close to be seen as seperate by the naked eye. Near them is the third member of the system, a faint star known as Proxima Centauri. Discovered in 1915, it is smaller than Alpha and Beta. Proxima (meaning nearest) it is slightly nearer to the Sun than the other stars in this triple star system.

The distance from the sun to Proxima Centauri, stated in five sources as a fact, has been found consistent. The distance to Proxima Centauri is 4.3 light-years.

## How many earth years would it take to reach andromeda galaxy?

The distance of Andromeda galaxy from Earth is 2.537 million light years and hence it will take that many years for light to travel to Andromeda galaxy.

However, to leave planet Earth, an escape velocity of 11.2 km/s or 40,320 km/h is required and a speed of 42.1 km/s is required to escape the Sun's gravity and exit the Solar System). As these are very small fraction of velocity of light, the time taken will be much much more than billions of years.

With a continuous acceleration of #1g# for #14# years followed by a continuous deceleration of #1g# for #14# years you could reach the Andromeda Galaxy. But.

#### Explanation:

(1) Sustaining an acceleration/deceleration of #1g# for #28# years would take a lot of energy/fuel.

(2) At the speeds you would reach, the radiation caused by encountering interstellar particles would probably be fatal unless some kind of powerful shield could be developed.

(3) While you would only experience #28# years (due to the effects of special relativity), the length of time experienced by observers at home would be about #2.5# million years. So what's the point of your journey anyway?