Distances to stars
Sizes and distances in the Universe are so enormous, we measure them in light years.
When we look at stars, some seem brighter than others. This may be because these stars really are brighter, or because they are closer than the other stars,. Closer stars seem brighter than stars that are further away.
To measure the distance to another star, scientists compare it's brightness with another similar star a known distance away. They need to know:
- how bright each star really is - this is the real brightness
- how bright they appear to be - this is the relative brightness
- how far away one of the stars is
These measurements are used to compare the distance of both stars. The unknown distance can then be calculated. This method can only give an estimate of the distance because it relies on assuming that similar types of star have the same real brightness. It is also based on the distance to one of the stars, which we do not know precisely.
It has always been difficult to make accurate observations of stars. The night sky can be obscured by clouds, must, fog or rain. Light from cities and towns, called light pollution, makes it hard to see the stars. Measurements are now more accurate.
Using parallax to measure distance
Parallax: The angle between two imaginary lines from two different observation points on Earth to an object such as a star or planet, used to measure the distance to the object.
As the Earth moves around the Sun, we look at nearby stars from a different angle. The nearby stars from a different angle. The nearby star seems to stay in the same place. The stars behind it seem to change their position. This parallax is used to measure distances to nearby stars.
If an object is nearby, the parallax is greater. If it is further away, the parallax is smaller.
There are several reasons why it is hard to be sure that parallax measurements are correct. For example:
- it is hard to get enough light from very dim stars and galaxies. Space telescopes like Hubble have helped with measurements like these.
- at long distances, parallax measurements are too tiny to measure accurately.
- we are not making direct measurements. Some information is obtained by comparing readings with readings taken from other stars.
Fusion in stars
The Sun gives out huge amounts of energy in the form of heat and light. It's energy comes from hydrogen. Hydrogen nuclei are jammed together so hard that they combine in pairs to form a different element, helium. This process releases loads of energy and is called nuclear fusion.
In a star, the temperature is so high and the density so great that nuclei collide at enormous speeds. When nuclei hit each other, two or more nuclei are crushed together in a fusion reaction to make a heavier nucleus. This means a different element forms. A nuclear fusion reasons give out heat and light energy.
Of all the atoms in the sun, 94% are hydrogen. In the Sun, the hydrogen nuclei fuse to make a helium nucleus plus heat and light energy. Hydrogen fusion is the source of the light and heat that we receive from the sun.
Some stars are much larger than others but all new stars contain hydrogen. As stars get older, fusion reactions in them change. Hydrogen nuclei fuse to make helium, then helium nuclei fuse, making beryllium and carbon. Other nuclear fusion reactions in different stars explain why we see other elements, as heavy as iron.
Elements in the Sun
Scientists had to find an explanation for how elements heavier than iron formed. Stars ore than eight times heavier than iron formed. Stars more than eight times heavier than our Sun end their life as a massive explosion called a supernova.
A supernova is an explosion of a large star at the end of it's life.
The explosion is so massive, lots of different elements are formed. Supernovas create all elements heavier than iron and spread them as dust and particles throughout the galaxy.
Analysing light from the Sun shows it contains hydrogen and helium. It also contains about 2% heavier elements like silicon and magnesium. This is unexpected as the sun still uses hydrogen as it's main energy source. The fusion reactions needed to make silicon and magnesium are not happening in the sun.
Scientists had to think creatively to explain this data from the Sun. Their explanation was that the Sun formed from dust and particles already present in the galaxy. The massive clouds of dust that form new stars contain particles from older stars that no longer exist. This means that the stars we see now contain elements that were formed in older stars that existed billions of years ago.
Moving galaxies and redshift
Starlight coming from galaxies to Earth changes during it's journey. The wavelength of the light wave increases, in just the same way as the wavelength of the sound waves from the siren of a fire engine zooming away from you. This stretching of the light waves is evidence that distant galaxies are moving away from us, very fast.
When light from stars in galaxies beyond our own is analysed, it appears redder than the light from similar stars in our own galaxy. Red light has a longer wavelength than blue light. This change in colour is caused by the stretching of light waves, and this is called redshift.
Light from nearly all galaxies is redshifted. Light from galaxies further away from us has a greater redshift. The amount of redshift depends on how fast the galaxy is moving away. It increases with the distance of the galaxy from us which tells us that the further away a galaxy is, the faster it moves away from us.
You cannot see the redshift using a telescope to look at distance galaxies. Instead, astronomers use an instrument called a spectrometer which analyses the light coming from galaxies. This gives enough information to tell them if the light has been redshifted. It can also measure the amount of redshift.
Explaining the motion of galaxies
When redshift measures were first made, scientists couldn't explain why galaxies were moving away from Earth. Assuming there is nothing special about Earth, the results must mean that nearly all galaxies are moving apart from one another.
One explanation is that all of space is expanding, like the surface of a balloon as it is inflated.
(As the balloon expands, all the dots move further apart).
This explanation accounts for the data from starlight, but when it was suggested it presented a completely new way of looking at the Universe. Scientists have extended the theory to predict what the Universe will be like in the future, and to consider what it was like in the past. They have also looked for more evidence to back up the theory.
Electromagnetic radiation created at the beginning of the Universe is still out there. It has been detected by the "COBE" satellite and analysed. The nature of the radiation and variations in it in different directions confirm the theory that the Universe expanded very rapidly from a point, billions of years ago.
How it all began
About 5 billion years ago, our Sun formed at the centre of a huge swirling cloud of gases and dust which collapsed on itself. A few million years later, the remaining matter swirling around the Sun formed into the planets, asteroids and moons. We think the Earth formed about 4.5 billion years ago.
As for the Universe itself, scientists believe the beginning of everything was when a rapid expansion started about 14 billion years ago. Matter and energy were flung outwards in what is called the Big Bang.
Scientists have found evidence for how the solar system formed:
- Telescopes in space can see how other new planetary systems form.
- Scientists have been able to date meteorites that have fallen to Earth. These rocks formed when the solar system formed. They are about 5 billion years old.
- Since objects in the solar system orbit in the same direction we think the whole solar system formed at the same time.
Evidence for how the Universe formed
The Big Bang theory predicted that an "echo" of the initial rapid expansion could be detected now, as radiation coming from all directions. The detection of this "cosmic background radiation" convinced many scientists that the Big Bang took place.
It was first detected over 40 years ago and reported in conferences and journals. Since then, other scientists have also detected the radiation, so the theory became widely accepted. But not all scientists agree.
The analysis of light from galaxies suggested that the Universe had been expanding for 14 billion years, but the data could be explained in different ways. Other theories about the Universe include the Steady State theory. This theory says that the Universe has always existed, and extra atoms are created as it expands. The Steady State theory explained the observes expansion but couldn't explain the discovery of cosmic background radiation.
Since most evidence fits in with the Big Bang theory, most scientists now believe this is correct.
We do not know what will happen billions of years in the future. The Universe may keep expanding, it could reach a fixed size, or it could collapse as a "Big Crunch".
The future of the Universe
It is hard to predict what will happen to the Universe, because of the difficulty of making reliable measurements. For example, we cannot measure the enormous distances accurately, nor the speed of the furthest galaxies.
Another thing we cannot measure accurately is the total mass of matter in the Universe. The total mass is crucial, because of the effect of gravity.
A total mass of Universe above the critical mass: Gravity starts to pull everything together again. The Universe reaches a maximum size then shrinks again.
A total mass of Universe equal to the critical mass: The Universe reaches a fixed size.
A total mass of Universe below the critical mass: Gravity is not strong enough to stop galaxies moving apart. The Universe expands forever.
Scientists think that the mass of the Universe is near the critical amount. More accurate measurements are needed for a definite answer.
Rocks on Earth
The Earth is constantly changing. Mountains are worn away by erosion. Eroded rock fragments are carried away and deposited as sediments, eventually forming new rock. The remains of animals and plants can form fossils if they are buried by sediments.
Some changes happen quickly, such as when a volcano erupts and the lava produces a new mountain or a crater. Movements of the Earth's crust over millions of years can cause rocks to fold, and can push rocks upwards to create new mountains.
Geologists study: the materials from which rocks are made, the different types of rocks and where they are found, the fossils contained in rocks, the shapes of mountains and of landscapes. Their findings provide evidence of how the Earth has changed.
Mountains, cliffs and other rock outcrops are constantly being broken down by weathering and eroded. This erosion is caused by: moving water (sea or rivers), glaciers (moving ice sheets), the wind (blowing particles away) and gravity (landslides and rock falls).
Weathering and erosion break down and remove the surface rock, changing the size and shape of mountains and valleys over millions of years.
How old is the Earth?
Weathered mountains become smaller and smoother, and valleys can become deep as rivers cut into the rock of the river bed. Eroded rock fragments are transported by the wind, water and ice, broken up further, and deposited on riverbeds and in the sea. This is called sedimentation. Over millions of years, the sediments are crushed together to form layers of new sedimentary rock. Where this rock forms, the sea will become shallower, as the rock beneath becomes thicker.
It is likely that erosion and sedimentation have occured all the time the Earth has existed. If no new rocks had been created, the large land masses would have been worn down to sea level. We now know that the Earth's outer layer - the crust - is constantly shifting.
Volcanoes create new mountains where molten rock escapes from under the Earth's crust and solidifies on the land surface or under the oceans.
Different parts of the Earth's crust move closer together over millions of years, pushing the rocks together and upwards to make new mountain ranges where the rocks collide.
Geologists can find and study mountains of all ages in different places in Earth. This means they can find examples of rock processes taking place today that can account for past changes.
Continental drift and Wegener's theory
Continents are huge land masses on Earth. They include Europe, Africa, Asia, North and South America, Antarica and Australia. A scientist called Wegener suggested that the continents were joined together in the past, and drifted apart over millions of years.
Alfred Wegener published his hypothesis of continental drift in 1915. His ideas were that the SHAPES of the continents looked as if they could interlock, similar FOSSIL TYPES were found on continents seperated by oceans and mountain chains made from the similar ROCKS appeared on the edges of different continents.
Wegener also thought that the forces causing the continents to move caused land to fold and deform into great mountain ranges along their edges.
Examples of fossils that were found include:
- Cynognathus (a dog-sized predatory reptile, 250 million years ago)
- Lystrosaurus (a pig-sized lizard-like herbivore, 250 million years ago)
- Mesosaurus (a freshwater reptile, 300 million years ago)
Opposition to Continental drift
Geologists study rocks and the Earth, but Wegener was not a geologist. Geologists believed that continents were in fixed positions, so if Wegener was right, many of their previous ideas were wrong.
1. Wegener could not explain how the continents moved.
2. The movement of the continents was too small to detect.
3. Data about rocks and fossils was not available for all continents.
4. There were simpler explanations for some of Wegener's ideas.
Wegener's ideas were so unexpected, scientists were sceptical. More evidence was needed to back up Wegener's hypothesis.
The spreading seafloor
The Earth's core is extremely hot and heats rock in the mantle. Some parts become hotter than others. This causes convection, which moves the rocks in the mantle. Sections of the Earth's crust above the mantle are forced apart, making the seafloor spread. This process is called seafloor spreading.
Every year, seafloors spread by several centimetres. We do not see a gap appear because fresh rock from the mantle comes to the surface to fill the space. This rock forms underwater in the mountains called oceanic ridges.
Over millions of years, the Earth's magnetic field changes its direction. Every time fresh rock comes to the surface, it is magnitised in the direction of the Earth's field at that time.
Scientists studying the magnetism of rocks in the oceanic ridges found that they contained strips of rock magnetised in opposite directions. This is evidence that the seabed is constantly forming in strips over millions of years. The hypothesis of continental drift explained the fossil and rock records better than previous explanations. It could not explain how and why the continents moved. It was not correct but it was not abandoned until scientists developed the theory of plate tectonics, which explained the data better.
The solid tectonic plates float on top of semi-solid rocks below the crust. The place where two plates meet is called the plate boundary. Earthquakes, volcanoes and new mountains are found at plate boundaries.
Tectonic plates move very slowly (a few centimetres each year), either pulling apart, sliding past each other or pushing together.
Volcanoes occur when plates move apart making a gap in the Earth's crust. Magma (liquid rock) is forced through cracks in the surface and piles up, forming volcanic mountains.
Mountains form when two tectonic plates move together and one plate is forced under the other. Volcanoes may occur.
Fold mountains form where tectonic plates made from similar rock densities push together. This lifts and folds the land.
Most earthquakes occur where the tectonic plates suddenly slide past each other, releasing energy in a sudden jerk.
The rock cycle
The rock cycle depends on the movement of tectonic plates.
- Mountains are weathered and fragments of rock break off.
- Rock fragments are transported away and deposited on the seabed as sediments.
- At plate boundaries, movement of the tectonic plates can force one plate beneath the other. When an oceanic plate is forced below the land, the sediments are dragged down as well.
- As the plate moves deeper into the mantle, the rock melts and becomes magma.
- Pressure can cause magma to rise. This magma either solidifies into rock beneath the land surface or escapes as a volcanic eruption to form new mountains.
Plate tectonics is a fairly new theory (1960s). It explains data such as the location of mountains/volcanoes/earthquakes, the shape of continents and the ages of mountains. Plate tectonics explains things that are not obviously related, such as why the direction of magmetism varies in rocks of different ages. It can also be used to predict where earthquakes are likely to occur.
Our Earth is made from several separate layers. The CRUST is a layer of solid rock about 30km thick. The MANTLE is a layer of semi-solid rock about 2900km thick. The OUTER CORE is a layer or liquid nickel and iron about 2200km thick and the INNER CORE is solid nickel and iron about 1250km thick.
The sudden movement of tectonic plates in an earthquake produces shock waves, known as seismic waves. These waves travel through the Earth. There are two main types of seismic wave.
P-waves travel only through solids and liquids.
S-waves only travel through solids.
Instruments on the Earth's surface can detect the waves after an earthquae. Studies of seismic data received by instruments in different parts of the world show that, P-waves arrive at detectors before S-waves. This means that P-waves travel faster than S-waves. In some places, P-waves arrive but S-waves do not. This suggests that some of the interior of the Earth is liquid. The speed of a wave depends on the path it takes through the Earth. This suggests that the Earth is made from layers of different materials.
Seismic waves and the Earth's structure
Scientists analyse the speeds of seismic waves by measuring the time it takes the different waves from an earthquake to reach detectors all over the world. Thy use what they know about how waves travel in different materials to explain their observations.
Seismic waves follow curved paths in the core and mantle. As seismic waves travel faster in denser materials, this tells us the Earth's density increases with depth.
There is a shadow zone where very few P-waves are detected. This is because the P-waves change direction abruptly at the boundary between the mantle and the core. P-waves travel more slowly in the core, so the mantle and core must have different densities.
No S-waves are detected on the opposite side of the Earth to an earthquake. As S-waves cannot pass through a liquid, this suggests that part of the Earth's core is a liquid.
P-waves are longitudinal waves. The vibration moves backwards and forwards along the direction the wave travels, squashing and stretching the material. P-waves are sometimes called pressure, push or primary waves. Sound waves are also longitudinal waves. S-waves are transverse waves. The vibrations move at right angles to the direction the waves travel in. S-waves are sometimes called secondary or shake waves. Light is also a transverse wave.
What a wave is
When you srop a stone in a puddle, there is a splash when te stone hits the water. Ripples spread outwards on the water's surface. The ripples carry energy from the impact of the stone. As each ripple passes, the water returns to where it was before.
These ripples are a type of water wave. A wave is a series of disturbances that carry energy in the direction of the wave, without transferring matter. For wave motion, we use the equation:
distance = wave speed x time
This can be rearranged to find out how fast a wave travels:
wave speed (m/s) = distance travelled(m) / time taken(s)
For example, if the ripple travels 0.5m in 5s, then we can calculate its speed.
wave speed = distance / time taken
wave speed = 0.5m / 5s = 0.1m/s
Vibrations cause regular cycles of disturbances. This is how waves are created. You can see and feel the vibrations causing sound waves. For example, the strings of a guitar vibrate white a note is being played and the voice box in your throat vibrates while you speak. An oscilloscope is a machine that displays waves on a screen. A grid on the screen lets you compare the wavelength and amplitude of waves.
A sound is louder is it has a larger amplitude.
A sound is higher pitched if it has a shorter wavelength.
The scale on an oscilloscope is used to measure a wavelength or amplitude. Wavelengths are measured in metres, but you do not need to know the units of amplitude.
If the distance in metres is 0.1m, and the vertical height is 3.5,the wavelength is 0.35 (0.1mx3.5)
If the vertical height is 5, and the amplitude is 2, the amplitude is 10 units (5 x 2)
Measure a whole wavelength from one point in a cycle to the same point in the next cycle, for example from peak to peak. Don't measure half the wavelength by mistake.
The wave equation
The number of waves passing a point every second is called the FREQUENCY of the waves. This is the same as the number of waves produced by a vibration every second. Frequency is measured in hertz, 1 hertz (Hz) equals 1 wave per second.
The length of each wave (for example from crest to crest) is the WAVELENGTH.
The speed is given by:
wave speed (metres per second, m/s) = frequency (hertz, Hz) x wavelength (metres, m)
This is called the wave equation.
All waves obey the wave equation. For example, the frequency of a particular sound wave is 440Hz, and its wavelength is 0.75m. Using the wave equation, we can calculate its speed.
speed = frequency x wavelength = 440 x 0.75 = 330m/s
Using the wave equation
wave speed (m/s) = distance travelled(m) / time taken(s)
wave speed (m/s) = frequency(Hz) x wavelength(m)
One consequence of the wave equation is that if the speed of a wave stays constant, then changing the frequency will change the wavelength.
All sound waves, for example, travel at the same-speed in air. High-pitched sounds have higher frequencies than low-pitched sounds. This means that the wavelength of a high sound must be shorter than the wavelength of a low sound for the speed to be the same.
The higher the frequency, the shorter the wavelength. The wavelength is inversely proportional to the frequency. This is true for all waves.
Our solar system
The solar system has the star called the Sun at its centre, it included all the objects that travel around the Sun in paths called orbits. These objects are different sizes and have different paths:
- Planets: The eight spherical planets travel around the Sun in near-circular orbits. The Earth is one of them.
- Dwarf planets: The five known dwarf planets are much smaller than planets. They also travel round the sun in near-circular orbits.
- Asteroids: These are lumpy rocks in near-circular orbits.
- Comets: These objects have very elongated orbits that stretch far away from the sun and may also approach very near to it.
- Moons: These are balls of rock that orbit a planet in near-circular orbits.
The Sun contains over 99% of the solar system's mass. In order of size, the next largest objects are the planets, then dwarf planets, comets, asteroids and moons.
A billion = 1,000,000,000
The Universe contains everything that exists. It contains thousands of millions of galaxies separated by enormous distances. Each galaxy contains thousands of millions of stars. The Sun, which is our nearest star, is just one of the thousands of millions of stars in our galaxy, the Milky Way.
Since distances are so large in the Universe, astronomers measure distance using light years. A light-year is the distance light travels in one year - 9.5 million million kilometres.
It is too far for people to travel to the stars. The only way we can find out about them is from the light and other forms of radiation they send out. Distant stars are very faint, or invisible to the naked eye. Scientists have to make predictions and hypothesis'.
A hypothesis becomes a theory if scientists test it over a period of time and their data keeps matching the hypothesis. Now, we have several accepted theories explaining why a star's brightness varies.
Proxima Centauri is our next nearest star. The light from it takes 4 years to reach us so we se it tonight as it was four years ago because it is four light years away.