Category Archives: Chapter 9 – Stars and Galaxies

Characteristics of stars

A star is a massive ball of plasma that emits light throughout the universe. While there is only one star in our solar system, there are billions upon billions of stars throughout our galaxy and exponentially more in the billions of galaxies in the universe. A star can be defined by five basic characteristics: brightness, color, surface temperature, size and mass.

Two characteristics define brightness: luminosity and magnitude. Luminosity is the amount of light that a star radiates. The size of the star and its surface temperature determine its luminosity. Apparent magnitude of a star is its perceived brightness, factoring in size and distance, while absolute magnitude is its true brightness irrespective of its distance from earth.

A star’s color depends on its surface temperature. Cooler stars tend to be redder in color, while hotter stars have a bluer appearance. Stars in the mid ranges are white or yellow, such as our sun. Stars can also blend colors, such as red-orange stars or blue-white stars.

Surface Temperature
Astronomers measure a star’s temperature on the Kelvin scale. Zero degrees on the Kelvin scale is theoretically absolute and is equal to -273.15 degrees Celsius. The coolest, reddest stars are approximately 2,500 K, while the hottest stars can reach 50,000 K. Our sun is about 5,500 K.

Astronomers measure the size of a given star in terms of our own sun’s radius. Thus, a star that measure 1 solar radii would be the same size as our sun. The star Rigel, which is much larger than our sun, measures 78 solar radii. A star’s size, along with its surface temperature, will determine its luminosity.

A star’s mass is also measured in terms of our own sun, with 1 equal to the size of our sun. For instance, Rigel, which is much larger than our sun, has a mass of 3.5 solar masses. Two stars of a similar size may not necessarily have the same mass, as stars can vary greatly in density.


Black Holes, Neutron Stars, White Dwarfs, Space and Time


Life and death of stars


Structure of the sun

The sun is the centre of our solar system. It is a blazing ball of gas which is the source of heat and light. Its great mass creates strong gravity to hold the nine planets in its orbits. The Sun contains more than 98% of all the matters in the Solar System.

Our Sun is an average star of yellow-white color, which indicates it is of medium age and temperature.

The Photosphere
The photosphere is the bright visible surface of the Sun. It is a shell of hot gas several hundred kilometers thick. Above the photosphere is a thick layer of cooler gas several thousand kilometers thick. When light passes through this layer, an absorption spectrum of the Sun is formed.

Sunspots occur in elaborate groups. Some are large enough to be visible to the naked eye. Sunspots are dark in appearance compared to the bright solar disk. The sunspot is divided into two regions. The dark inner region is known as the umbra while the light outer region is called the penumbra. The number of sunspots visible changes through time. A spot may be visible for several days or weeks before it is dissolved.

The Chromosphere
The Chromosphere is about 1500 km thick. It reveals a reddish glow around the rim of the Moon during a solar eclipse. The spectrum we see is referred to as the flash spectrum. The chromosphere is best studied during a solar eclipse but it can also be studied by the aid of an instrument called coronagraph which creates an artificial eclipse.

The Corona
The corona, meaning crown in Latin, is above the chromosphere. Its total brightness adds up the brightness of a full moon, yet its temperature is incredibly high. The corona changes month to month. Part of the corona is the Sun’s scattered light, another part are few bright emission lines which elements do not occur naturally on Earth. The hypothetical element is known as “coronium”.

The Solar Wind
The solar wind is the flow of protons, electrons and helium nuclei. It is the outer most extension of the sun’s atmosphere. The particles of the solar wind travels along open magnetic field lines through coronal holes but not from all parts of the Sun.


Birth of a star?

Stellar nebula
Moments after the Big Bang, energy begins to condense into matter, protons and neutrons are formed, and then the first element (hydrogen) is formed. Hundreds of millions of years later in stellar nebulae, the hydrogen gas clouds coalesce and, under gravity, form protostars. Nuclear fusion processes begin converting hydrogen into helium. One example of a stellar nursery is the Eagle Nebula .

Average star
An average or medium star is less that 3 times the mass of the Sun. Stars are powered by nuclear fusion in their cores, mostly converting hydrogen into helium and liberating tremendous amounts of energy.

Massive star
Massive stars, more than 3 times the mass of the Sun, mostly convert hydrogen into helium. Rigel is the brightest star in the constellation called Orion and one of the brightest stars in the sky. It is a blue (very hot) supergiant, over 100 times bigger than the Sun.

Red giant
As medium sized stars exhaust their hydrogen content, they expand up to 100 times their original size to become red giants. The nuclear fusion reactions occurring within a red giant are H > He and He > C. Our Sun will follow this path over the next 5 billion years . This red giant is Aldebaran in the constellation Taurus.

Super red giant
Supergiants are the element factories of our universe. The nuclear fusion reactions occurring are H > He, He > C, C > Ne, Ne > O, O > Si and Si > Fe. Betelgeuse in the constellation Orion is a super red giant. It is about 20 times as massive as the Sun. The lifetime of this type of star is relatively short by comparison with the Sun – millions of years as opposed to billions of years.

Planetary nebula
A planetary nebula is a huge shell of gas and dust ejected during the last stage (red giant) of the life of a medium star. Elements such as helium, carbon, oxygen, nitrogen, neon and smaller amounts of heavier elements are present. Planetary nebulae play an important part in the chemical evolution of the galaxy, allowing these elements to be returned to the interstellar medium. The remains of the carbon core of a red giant evolve into a white dwarf star. The Eskimo nNebula in Gemini is a good example.

From the cataclysmic explosion of the supernova, the heavier elements form. The supernova is the final stage in the life of massive stars. The outer region of the star collapses and it Instantly rebounds off the inner core in a cataclysmic explosion. The extremely high level of energy allows further fusion reactions to occur, producing heavy elements like gold, silver and uranium. The supernova image shows Tycho’s Supernova Remnant – this expanding gas cloud is all that remains after a star went supernova. In 1572, Danish astronomer Tycho Brahe noticed the presence of a ‘new’ bright light in the night sky and recorded its position and intensity in his writings. It has been named in his honour.

White dwarf
A white dwarf is a small, very dense, hot star that is made mostly of carbon. These faint stars are what remain after a red giant star loses its outer layers. They are about the size of the Earth and will eventually lose their heat to become a cold, dark black dwarf. The sun will eventually turn into a white dwarf and then a black dwarf.

Neutron star
Stars with a mass between 1.5 and 3 times the mass of the sun will end up as neutron stars. A neutron star is a very small, super-dense star that is composed mostly of tightly packed neutrons. A rapidly spinning neutron star is known as a pulsar.

Black hole
Black holes are all that remain after stars with masses over 3 times that of the sun supernova. A black hole is a massive object (or region) in space that is so dense that, within a certain radius (the Schwarzschild radius determines the event horizon), its gravitational field does not let anything escape from it –


Sun spots and solar prominence

Sunspots appear as dark patches. They appear dark because they are cooler than other parts of the surface. Sunspots do not last more than a few weeks however every 11 years, the Sun is very active with many sunspots.The sunspots are magnetically active spots which are caused by magnetic disturbances deep inside the Sun. These disturbances can cause changes in the Earth’s climate, for example, extreme drought.

Prominences or ‘giant flames’ are also seen in the photosphere and can extend thousands of kilometres from the surface. Surface gases like Helium and Hydrogen escape to outer space also carry a stream of energetic , electrically charged particles which also cause wind known as solar wind. Solar wind can affect satellite, radio, television, telegraph and telephone communication. The particles also affect the Earth climate and also cause aurora which ia a phenomenon in which the sky in the polar regions appears colourful.


The amazing solar flares