What is the difference between stars planets and galaxies

The Moon cal also be considered a satellite. What is the difference between: a star, moon, planet, galaxy, universe, and a satellite? Astronomy Introduction to Astronomy Astronomy Basics. Phillip E. Mar 5, They are all names for objects in the Universe. Explanation: A star is a sun which produces energy from nuclear fusion. The universe is all of the galaxies and other objects that we know of.

Related questions Why do astronomers use scientific notation to describe sizes? What is astronomy? The corona sometimes looks like streamers or plumes, but its shape changes from eclipse to eclipse, although it will not usually show any changes during the few minutes of totality that characterise a typical total eclipse. The corona is very hot temperatures of several million kelvin are not unusual but it is so thin that its pearly white light is very faint compared with the light from the photosphere.

The bright light from the Sun's photosphere is scattered by the Earth's atmosphere. This makes the sky blue and generally rather bright. As a result, the much fainter light from the corona can't be seen rather as the light from a dim torch is unnoticeable on a bright sunny day.

Sometimes in eclipses observers also see prominences - great spurts of hot material at the edge of the Sun, extending outwards from the solar surface for many thousands of kilometres. Prominences and the changing shape of the corona indicate that the Sun is an active body, not just a quietly glowing source of light.

When the Sun is observed with instruments that can detect electromagnetic radiation other than visible light, it is possible to see the full extent of the activity of the Sun. Prominences, sunspots and other features of the Sun seen at different wavelengths are indicative of active regions, generally caused by the Sun's magnetic field, which influences the flow of hot gaseous material on the Sun.

Sudden changes to the magnetic field in the corona are thought to be responsible for flares, one of the most energetic of all solar phenomena, which emit bursts of radiation of all wavelengths, from radio waves to gamma-rays as well as energetic particles such as fast-moving protons and electrons. The Sun is a typical star and only appears much brighter than other stars because they are so much further away. Astronomers can use this to deduce the actual distances to stars.

One important observation makes this easier: namely, stars that are the same size and colour give out the same amount of light. So, if astronomers observe two stars of exactly the same colour, they can start by assuming they are the same size and therefore they must be giving out the same amount of light.

If one looks fainter, it must be further away. By measuring the amount of light entering a telescope from each star, astronomers can work out just how much further away one star is than the other. Figure 10 shows the principle.

Stars A and B give out the same amount of light, but B is at twice the distance of A, so its light is more spread out by the time it reaches an observer on Earth.

When a number is multiplied by itself, the result is called the square of that number. For example, multiplying three by three gives nine. Nine is said to be 'the square of three' or 'three squared'. Note that 3 3 can also be described as 'three to the power of three'. What is the general rule for describing how the apparent brightness of a star diminishes with distance? The general rule is that the apparent brightness diminishes with the square of the distance.

Squares can also be worked out backwards so that as nine is three squared, three is the square root of nine, written , and as 27 is three cubed, three is the cube root of 27, thus.

The apparent brightness of a star or any other luminous object is therefore said to obey an inverse square law. In practice, it is not quite so easy to measure distance, because some stars are the same colour but different sizes and so give out different amounts of light - but the general principle of 'faint means far' underlies many of the techniques for measuring distances. Figure 11 shows that stars have different colours.

These colours are related to the temperatures of the stars. The Sun is yellowish, with a photospheric temperature of about K. Bluish-white stars are hotter than the Sun and orange-red stars are cooler.

Stellar temperatures range from less than K to over 40 K. Stars come in a range of sizes, masses and luminosities. White dwarfs are only around the size of the Earth whereas some red giants are so large that if placed at the location of the Sun, they would engulf the Earth! The masses of stars, however, cover a much smaller range. The least massive are around ten percent of the mass of the Sun and the most massive around a hundred solar masses. The mass of the Sun, 1 solar mass, denoted , provides another commonly used unit in astronomy.

The subscript symbol represents the Sun, so the radius of the Sun is and the luminosity the total power output of the Sun is. The luminosities of stars range from less than a thousandth of the solar luminosity to greater than. Figure 12 shows examples of different types of stars. A wide variety of combinations of properties are found, such as small, cool, faint red dwarfs and large, hot, highly luminous supergiants.

However, some types are more common than others and not all possible combinations of these properties are found among the stars that have been observed. The Sun is one of about a hundred billion 10 11 stars in our galaxy.

It is difficult to determine the structure of the Galaxy as we are located inside it. As well as stars, it contains vast clouds of gas and dust, which can obscure our view in certain directions. However, if you observe the night sky from a dark site on a clear moonless night, you will see the Milky Way, a band of light circling the sky, that comes from many faint stars that cannot be individually distinguished Figure It reveals the most obvious characteristic of our galaxy, that it has a flattened shape.

Careful analysis of the distances and motions of stars in space are required to reveal the true nature of our galaxy, called the Milky Way galaxy. The human eye is extremely sensitive, but even with the aid of a telescope it is not ideal as an astronomical detector, because it does not record images.

Until the use of photography in the late nineteenth century, astronomers recorded their observations with drawings made at the telescope.

Despite the fact that photographic plates were much less sensitive than the human eye, they had one additional critical advantage - they could accumulate the light from a faint object for as long as a telescope could track it the human eye retains an image for less than a tenth of a second. Photographs, and more recently electronic imaging detectors, reveal a huge variety of galaxies Figure They revealed that what appeared as faint smudges of light to the eye were in fact vast systems of stars like our own galaxy.

Our galaxy is sometimes referred to as 'the Galaxy' with a capital 'G' to distinguish it. Galaxies are not distributed uniformly in space. Our own galaxy is a member of a small group of about 40 galaxies. Larger galaxy clusters may have more than a thousand members see Figure 15 and these clusters themselves appear to be arranged into even larger structures.

Our understanding of the Universe is, not surprisingly, derived largely from the light emitted by stars and galaxies. However, our understanding of the properties and evolution of these stars and galaxies comes from applying scientific principles and mathematical models. As new observational techniques developed and this understanding grew it became more apparent that the objects that can be seen represent only a fraction of the matter in the Universe.

The majority of matter, called dark matter, is not visible but is required to understand the properties of the Universe. Some dark matter may simply be in the form of dead stars, but most appears not to be made up of the familiar elements but of some so far unknown constituents. This material is called non-baryonic dark matter. Figure 16 indicates what is currently thought to be the material composition of the Universe. How did the atoms in our bodies, the Earth, the Sun and other astronomical objects originate?

This question is intimately tied with one of the most fundamental questions in science 'How was the Universe formed? Only the simplest atoms were present in the early stages of the Universe. Even now, the composition of normal matter in the Universe is dominated by hydrogen and helium. The heavier elements have been produced in nuclear reactions within stars. If the nuclear reactions in stars occur close to the centre where the temperatures and pressures are highest, how can these heavy elements escape to make their way into the gas and dust clouds in the Galaxy and ultimately into planets and us?

The answer lies in the details of the evolution of stars. The structure of stars changes as they age and some end their lives in catastrophic explosions that distribute much of their mass into the surrounding space and increase the fraction of heavier elements that can be incorporated into new stars. If the Sun formed from clouds of gas and dust in the Galaxy why does it contain only half the amount of heavy elements now present in the Galaxy?

The Sun formed when the Galaxy was considerably younger around 4. As time passes more and more stars complete their life cycles and so the proportion of heavy elements increases. The Sun therefore formed when there were fewer heavy elements in the Galaxy. Table 2 lists the fraction by mass of the most common elements in the photosphere of the Sun compared with those in the Earth and a human body. As you would expect, the elements that are most common in the Sun are those that are most commonly produced in the nuclear reactions in stars.

However, if the Earth formed at around the same time as the Sun why does it have such a different composition? The most abundant atoms in the Universe are only minor constituents of the Earth.

The answer lies in the way in which these elements are combined in molecules that formed the building blocks of the planets. The Earth is a rocky body, so elements that form minerals and rocks oxygen, silicon and metals such as iron and magnesium are the most common. Helium is an inert gas as are neon and argon , which means that it does not react with other atoms to form molecules, and therefore does not contribute significantly to the rocky material of the Earth.

The gas giant planet Jupiter has a composition much closer to that of the Sun; its vast atmosphere is composed of mainly hydrogen and helium.

The elemental composition of the human body is dominated by hydrogen, carbon and oxygen, three of the four most abundant atoms in the Sun. Water is formed from hydrogen and oxygen, and carbon is the key to forming highly complex molecules. These organic carbon containing molecules provide the framework for constructing the complex structures present in the human body and for carrying the information that allows humans to grow and reproduce one of the definitions of life.

Helium is the second most abundant element in the Universe but is not a significant component of the human body. Why is this? Because helium is an inert gas it does not form into molecules that are present in the human body. It is beyond the scope of this short discussion to investigate the requirements for these complex molecules that are essential for life as we know it to exist. Nevertheless understanding how those conditions are satisfied on the Earth is the first step in attempting to find environments elsewhere where life may exist.

Current views of different parts of the Universe offer a wonderful spectacle, but all astronomers know that, as they peer across vast cosmic spaces, they also look back over great reaches of cosmic time.

This is an unavoidable consequence of the finite speed of light. The light seen today from the most distant observable galaxies was emitted over 12 billion years ago. The earliest signals of any kind that can be detected a particular kind of microwave radiation that is the remnant of the Big Bang that is believed to have formed the Universe originated over 13 billion years ago.

The Sun and Earth were formed around 4. Only in the last million years of this vast timescale, did humans evolve on Earth Figure 17 , and only in the last century have they been able, in theory, to communicate their presence to other possible inhabitants of our galaxy.

One of the most exciting recent developments in astronomy has been the detection of planetary systems around other stars. Many astronomers believe firm evidence for the presence of extraterrestrial life, either on another planet in our Solar System or orbiting a different star, will be found during our lifetimes.

The prospects for finding extraterrestrial intelligent life appear to be much more remote. Question 1 What would be the diameter of the Sun if it was represented in the model of the Solar System described in Table 1? Question 2 Distances to nearby stars are a few light-years. Calculate how long it takes light to reach the Earth from the Sun and hence explain why the light-year is not a useful unit for measuring the distance to the very nearest star, the Sun.

Question 3 Two stars, X and Y, have the same size and colour but X is four times further away from an observer on Earth than Y. How will the apparent brightness of the stars compare to the observer?

Question 4 The majority of the Earth's rocky surface is comprised of silicate rocks compounds containing silicon and oxygen together with metals such as aluminium, calcium, magnesium and iron. The core of the Earth is believed to be predominantly composed of iron. What evidence is there in Table 2 to support this? Question 1 The Sun has a diameter of 1.

The scale of the model is 1 cm to km. There is no fruit large enough to represent it in the model! This is a tiny fraction of a year which is over 30 million seconds so a light-year is not an appropriate unit to measure distances within the Solar System.

Question 4 The composition of the crust is dominated by silicon and oxygen whereas the whole Earth has a smaller proportion of these elements indicating that the proportion of rocky material is lower overall. The proportion of iron is much higher in the whole Earth than in the crust, indicating that the iron is concentrated towards the centre.

In the first part, you might have found that the table-tennis ball did not move exactly in a straight line. This might happen if the surface was not level, or if either the ball or the table were not completely smooth.

If you inadvertently spun the ball, and there was some friction between the ball and table, that would also drive it into a curved path. In the second part, the cork flies off in the direction it was heading, and falls in a curved path towards the Earth. It is being pulled downward by gravity. If there was no gravity it would move horizontally in a straight line. This free course provided an introduction to studying Learn about galaxies, stars and planets.

It took you through a series of exercises designed to develop your approach to study and learning at a distance, and helped to improve your confidence as an independent learner. The material acknowledged below is Proprietary, used under licence and not subject to Creative Commons licence. See terms and conditions. West, M. ESO, Chile ;.

If reading this text has inspired you to learn more, you may be interested in joining the millions of people who discover our free learning resources and qualifications by visiting The Open University - www. This free course is adapted from a former Open University course called 'Galaxies, stars and planets S '. Printable page generated Sunday, 14 Nov , Use 'Print preview' to check the number of pages and printer settings.

Print functionality varies between browsers. Printable page generated Sunday, 14 Nov , Galaxies, like solar systems, are held together by gravity. In galaxies, the solar systems are separated by vast sections of mostly empty space. The galaxy that contains the Earth and its solar system is called the Milky Way. This galaxy is thought to contain more than billion different stars. Solar systems orbit around their galaxies just as planets orbit around their suns.

It takes the Earth's solar system roughly to million years to complete its orbit. The universe is the largest of these three astronomical concepts. All things, including galaxies and solar systems, are included within the realm of the universe. Although everything known to man is contained within the universe, scientists believe the universe to be continually expanding.

This is thought to be a result of the big bang, the massive explosion of super-condensed matter that created the universe and all things contained within. Size is the major difference between the universe, galaxies and solar systems. Other differences exist as well, however. Black holes are sections of space with intense gravitational pulls, from which not even light can escape.