Earth, stars, galaxies and space

The Earth and the Universe
1. Much is now known about the Earth and the place of the Earth in the Universe, for example:
  1. the diameter of the Earth is 12,800km (7,953 miles)
  2. the diameter of the Sun is 109 times that of the Earth’s
  3. the Earth is 150 million km (93 million miles) from the Sun
  4. the distance to the nearest star is four light years.
The Solar System
1. The Earth is just one of the eight planets orbiting the Sun, which is a star. The orbits all lie in the same plane, and the planets all go round in the same direction. There are many other members of our Solar System:
  1. asteroids are much smaller than planets, and orbit the Sun. Most of the asteroids are between the planets Mars and Jupiter, but some come close to the Earth
  2. moons orbit planets. Most are tiny. Only a few are as large as our Moon, which is nearly a sixth of the diameter of the Earth
  3. comets have different orbits to those of planets, spending much of their orbital time far from the Sun. Comets are similar in size to asteroids, but are made of dust and ice. The ice melts when the comet approaches the Sun, and forms the comet’s tail.
The Sun
1. Nearly all of the mass in our Solar System is in the Sun. The Sun is very large. Its diameter is 109 times the Earth's. The Sun is the source of nearly all the energy we receive. For many years, it was a mystery as to where this came from and this baffled the leading scientists. It is now understood that the nuclear fusion is the energy source. In nuclear fusion, smaller nuclei come together and form larger nuclei. For example hydrogen nuclei are joined together to make helium nuclei. This releases enormous amounts of energy.
2. hydrogen nucleus + hydrogen nucleus → helium nuclei
3. In stars larger than our Sun helium nuclei can be fused together to create larger atomic nuclei. As the Earth contains many of these larger atoms, like carbon, oxygen, iron, etc, scientists believe that our Solar System was made from the remains of an earlier star.
4. How stars and planets are formed
  1. As the gas falls together, it gets hot. A star forms when it is hot enough for a nuclear fusion reaction to start. This releases energy, and keeps the star hot. The outward pressure from the expanding hot gases is balanced by the force of the star's gravity. This happened about 5 billion years ago. This is quite recent in the history of the Universe, which is currently believed to be 14 billion years old.
Looking at the sky
1. The radiation that distant stars and galaxies produce gives us information about the distances to stars, and about how they are changing. In the future, this may allow us to find out if life exists on planets around some of these stars.
2. Everything we know about stars and galaxies has come from the light, and other radiations, that they give out. This has become more difficult to see from the Earth’s surface, as light pollution from towns and cities interferes with observations of the night sky.
3. Looking at the sky with the naked eye shows the Sun, Moon, stars, planets and a few cloudy patches called nebulae. When telescopes were invented and developed, astronomers could see that some of the nebulae were in fact groups of millions of stars. These are galaxies.
4. Parallax
  1. Powerful telescopes allowed astronomers to answer a question that had baffled scientists since the astronomer Copernicus (1473-1543) first suggested that the Earth moved around the Sun. If the Earth moves, you would expect to see a different view of the stars at different times of the year, in the same way as the room you are in looks slightly different if you move your head to one side. That is to say everything seems to move in the opposite direction to your head, but the objects close to you seem to move more. This effect is called parallax. So if the Earth was moving, why did the stars always look the same?
    1. The answer to the question was revealed by more powerful telescopes. These showed that nearby stars do seem to move from side to side and back every year when compared with very distant stars, but that the amount of movement is tiny.
      1. The second nearest star to us is Proxima Centauri. The Sun is the nearest. It seems to move through an angle of 1.5 seconds between January and June. As one second = 1/60 of a minute, and one minute = 1/60 of a degree, this tiny movement, which is less than a thousandth of the diameter of the Moon, needed powerful telescopes and accurate measurement to observe.
  2. In the last 200 years, it has become very difficult to make astronomical observations in industrialised countries such as the UK. This is not just because of cloudy weather or air pollution. It is due to the bright lights found in cities and towns, and on roads. This light pollution means that it is hard for many people to see more than a few of the very brightest stars at night.
5. Telescopes
  1. Telescopes are now placed in the few remote, dark places left on our planet, or out in orbit around the Earth.
  2. The Very Large Telescope is part of the Paranal Observatory that is built on top of the Cerro Paranalmountain, which is 2,635 m high, in the Atacama Desert in Chile.
  3. Telescopes in space, such as the Hubble Space Telescope, can observe the whole sky. They are above light pollution and above dust and clouds in the atmosphere. However, they are difficult and expensive to launch and maintain. If anything goes wrong, only astronauts can fix them.
6. Beyond our Solar System
  1. The Sun is 150 million km(93 million miles) from the Earth, but that’s a tiny distance compared with the distance to other stars, or other galaxies. Larger units of distance are used for these measurements. One popular measurement is the light-year.
7. Light-years
  1. A light-year is the distance light travels in a year. Light travels very fast (300,000 km/186,282 miles per second), and takes only about eight minutes to reach us from the Sun. It takes over four years to reach us from the next nearest star (Proxima Centauri), and 100,000 years to cross the Milky Way galaxy. We say that the distance to the next nearest star is four lightyears, and the diameter of the Milky Way is 100,000 light years.
  2. The most distant galaxies observed are about 13,000 million light-years away. However, measuring distances to other stars, and to very distant galaxies, is not easy, so the data is uncertain.
Measurement uncertainties
1. When initial distances to stars were being established more than one method was employed. After establishing distances of nearby stars using the parallax method, the 'brightness method' was used to approximate distances to further stars. Other methods were also used.
2. Each method had its own assumptions. For example, with the parallax method an assumption made is that during the total time in which the measurement is taking place, distance remains constant between the two stars.
3. As methods were reliant on each other, a certain level of uncertainty is found in the results
Galaxies
1. Galaxies contain thousands of millions of stars. For many years, it was thought that our galaxy, which is the Milky Way, was the only one that existed, and that the blurry nebulae that could be seen were clouds of dust and gas in the Milky Way.
2. Observations of many of these nebulae by astronomers such as Edwin Hubble showed they were in fact galaxies outside the Milky Way, and that distant galaxies are all moving away from us.
3. The beginning and end of the Universe
  1. Hubble’s observations led to the ‘Big Bang’ explanation of the beginning of the Universe, and set a date for this at 14,000 million years ago.
  2. There are many questions left unanswered about the beginning and end of the Universe. Observations suggest it contains a lot of ‘dark matter’ that cannot be seen, and this is not yet clearly understood.
  3. Perhaps the Universe will continue to expand in the way it is at the moment. Perhaps gravity will eventually win and pull all the fleeing galaxies back together again. Better observations of very distant galaxies and a better understanding of the mysterious ‘dark matter’ are needed before these will be understood.
4. Hubble’s Law- Higher tier
  1. The astronomer Edwin Hubble (1889-1953) measured the distance to many galaxies, and also the speeds with which they are moving away from us. He found a strong correlation between these factors.
  2. This correlation is summed up in Hubble’s Law which says that the speed at which a galaxy moves away from us is proportional to its distance from us
    1. As the Universe expands, galaxies that are already further apart will increase in separation even more, and so move away at higher speeds.
Age of the Universe
1. The development of powerful telescopes allowed astronomers to see distant galaxies. The light observed was shifted towards the red end of the spectrum. This phenomenon is known as red-shift. The degree to which light has been shifted indicates how fast the galaxies are moving away.
2. In general, the further away the galaxy is, the faster it is moving away from the Earth. The motions of the galaxies themselves suggest that space itself is expanding.
3. It is estimated that the Universe is approximately 13.7 billion years old. Evidence suggests that our Solar System formed around 4.5 billion years ago, so it is around one-third the age of the Universe.
4. The eventual fate of the Universe is hard to predict due to the uncertainty in measuring such large distances and studying motion of distant objects. A better idea of the mass of the Universe would lead to better predictions.

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