STARS, GALAXIES AND THE UNIVERSE

The beginning…

What are Stars?Stars are large balls of hot gas.They look small because they are a long way away, but in fact many are bigger and brighter than the Sun.The heat of the star is made in the centre by nuclear fusion reactions.There are lots of different colours and sizes of star.

A STAR IS BORN…

A STAR IS BORN…

1st Step:Stars form from nebulas -

Regions of concentrated dust and gas.

Gas and dust begin to collide, contract and heat up,

All due to gravity

2nd Step:As a nebula contracts, a small star is formed

Called a protostar.

Eventually, the protostar will begin nuclear fusion.

Hydrogen protons attract to each other and fuse together.

Nuclear fusion = Hydrogen into Helium

This necessary for stars to survive.

3rd Step:Star joins the main sequence.

90% of stars spend their life here.The mass of star determines its location on the main sequence.

Beginning of the End:Stars begin to die when they run out of hydrogen.

Gravity begins to take over.

Star begins to shrink; outer core of hydrogen begins to fuse.

The outer part of the Star gets bigger.

Beginning of the End:When a star gets bigger, it cools down and becomes a

Red giant.

Eventually, the star can fuse helium into other elements such as

Carbon, oxygen, and other heavier elements.

Beginning of the End:Once a star runs out of “fuel”, the star shrinks under its own gravity.

It can Turn into a white dwarf, neutron star, or black hole.

Death of Stars:What stars end up as depend on their mass.

Low and Medium mass stars

Planetary nebula --------- white dwarf.

High mass starsSupernova --------- neutron star or black hole.

Death of Stars: Low and Medium Mass

Main Sequence Star

Red Giant

Planetary Nebula

White Dwarf

Death of Stars: High MassMain Sequence Star

Red Super Giant

Supernova

Neutron Star Black Hole

High Mass Stars:Mass greater than 8x our sun

Create high mass elements such as iron

Neutron StarFormed if remaining star < 3x sun’s mass

Black HolesFormed if remaining star > 3x sun’s mass

Distances To The StarsStars are separated by vast distances.Astronomers use units called light years to measure the distance of starsA light-year is the distance that light travels in a vacuum in a yearProxima Centauri, is the closest star to the sun.

9,461,000,000,000 trillion km or 5,878,000,000,000 trillon miles. 

How are Stars Classified?

They are classified by their temperature (color) and

brightness.The H R Diagram showsthe temperature and brightness

of stars.

A star is made up of different elements in the form of gases. The inner layers are very dense and hot. But the outer layers are made up of cool gases. Elements in a star’s atmosphere absorb some of the light that radiates from the star. Because different elements absorb different wavelengths of light, astronomers can tell what elements a star is made of from the light they observe from the star. This is called the spectrum.

The spectrum consists of millions of colors, including red, orange, yellow, green, blue, indigo and violet. A hot solid object gives off a continuous spectrum—a spectrum that shows all colors. However the spectrum of a star is different. Astronomers use an instrument called a spectrograph to break a star’s light into a spectrum. This information tells astronomers information about the composition and temperature of a star.

Brightness of a Star.Absolute magnitude – how bright a star really is.Apparent magnitude – how bright a star looks from Earth.

Motion of stars.

Different types of stars.• We have learned that stars are classified by

their size, mass, brightness, color, temperature, spectrum and age. Some types of stars include main sequence stars, red giants, supergiants, and white dwarf stars. A star can be classified as one type of star early in its life cycle and hen be classified as another star when it gets older. The life cycle for an average star, such as our sun includes different stages. The first stage is star formation. The second and longest stage is the main sequence. The third stage a star can become a red giant or super red giant. The last stage is a white dwarf.

Main sequence stars are stars that are fusing hydrogen atoms to form helium atoms in their cores. Most of the stars in the universe — about 90 percent of them — are main sequence stars. The sun is a main sequence star. These stars can range from about a tenth of the mass of the sun to up to 200 times as massive.

Red Giants and Super Red Giants

A red giant star is a dying star in the last stages of stellar evolution. In only a few billion years, our own sun will turn into a red giant star, expand and engulf the inner planets, possibly even Earth.

Stars spend approximately a few thousand to 1 billion years as a red giant. Eventually, the helium in the core runs out and fusion stops. The star shrinks again until a new helium shell reaches the core. When the helium ignites, the outer layers of the star are blown off in huge clouds of gas  and dust known as planetary nebulae.The core continues to collapse in on itself. Smaller stars such as the sun end their lives as compact  white dwarfs.

Super Red Giants

Red supergiant stars are the largest stars in the Universe by volume (meaning they also have the greatest diameter), however, they are not necessarily - and almost never are - the largest stars by mass.

The star will remain a red giant until the core reaches a high enough temperature to begin fusion helium into carbon and oxygen. At this time the star shrinks down slightly into a yellow giant.

White Dwarf

White dwarfs are the burned-out cores of collapsed stars that, like dying embers, slowly cool and fade away. They are the remnants of low mass stars, among the dimmest objects observable in the Universe. They are low to medium (less than ten solar mass) Main Sequence stars which have burned through their reservoirs of both hydrogen and helium, passed through the giant phase, were not hot enough to ignite their carbon, puffed off their outer layers to form colorful planetary nebula, and then collapsed and cooled into small glowing coals. This beautiful Hubble Space Telescope image shows a nearby white dwarf, and the outer layers of the former star's atmosphere which have been blown away. The resultant planetary nebula will shine for the next 20,000 to 50,000 years, expanding outwards and fading slowly with time.

This beautiful Hubble Space Telescope image shows a nearby white dwarf, and the outer layers of the former star's atmosphere which have been blown away. The resultant planetary nebula will shine for the next 20,000 to 50,000 years, expanding outwards and fading slowly with time.

When massive stars die…A massive star is a star with a mass eight times greater than that of the Sun. It is difficult for stars to get this large, as a number of factors influence stellar development and these factors often limit size, but astronomers have been able to observe massive stars up to 150 times larger than the Sun, illustrating that it is possible under the right conditions. 

Massive stars use their hydrogen much faster than stars like the sun. Massive stars generate more energy and are very hot! Massive stars do not have long lives. At the end of its life the massive star may explode in a bright flash of light called a supernova.

Supernova explosionA blindingly bright star bursts into view in a corner of the night sky — it wasn't there just a few hours ago, but now it burns like a beacon.That bright star isn't actually a star, at least not anymore. The brilliant point of light is the explosion of a star that has reached the end of its life, otherwise known as a supernova.Supernovas can briefly outshine entire galaxies and radiate more energy than our sun will in its entire lifetime. They're also the primary source of heavy elements in the universe. According to NASA, supernovae are “the largest explosion that takes place in space.”

What happens after a supernova explosion…

A supernova is a cataclysmic event that occurs as a result of the final uncontrolled nuclear reactions in a very high mass star at the end of its life. The giant star explodes violently due to the collapse of its core, hurtling all or most of its material outward at extremely high velocity. In some cases, a supernova will produce more light, for several weeks following the explosion, than the entire galaxy in which it resides. The remains of this titanic explosion consist of an expanding debris cloud and possibly an imploded remnant of the core, such as a neutron star, pulsar, or black hole.

Neutron StarWhen stars four to eight times as massive as the sun explode in a violent supernova, their outer layers can blow off in an often-spectacular display, leaving behind a small, dense core that continues to collapse. Gravity presses the material in on itself so tightly that protons and electrons combine to make neutrons, yielding the name "neutron star."

Pulsar

A pulsar is a neutron star that emits beams of radiation that sweep through Earth's line of sight. Like a black hole, it is an endpoint to stellar evolution. The "pulses" of high-energy radiation we see from a pulsar are due to a misalignment of the neutron star's rotation axis and its magnetic axis

Black HolesA black hole is a place in space where gravity pulls so much that even light can not get out. The gravity is so strong because matter has been squeezed into a tiny space. This can happen when a star is dying.Because no light can get out, people can't see black holes. They are invisible. Space telescopes with special tools can help find black holes. The special tools can see how stars that are very close to black holes act differently than other stars.

Scientists can't directly observe black holes with telescopes that detect x-rays, light, or other forms of electromagnetic radiation. We can, however, infer the presence of black holes and study them by detecting their effect on other matter nearby. If a black hole passes through a cloud of interstellar matter, for example, it will draw matter inward in a process known as accretion. A similar process can occur if a normal star passes close to a black hole. In this case, the black hole can tear the star apart as it pulls it toward itself. As the attracted matter accelerates and heats up, it emits x-rays that radiate into space. Recent discoveries offer some tantalizing evidence that black holes have a dramatic influence on the neighborhoods around them - emitting powerful gamma ray bursts, devouring nearby stars, and spurring the growth of new stars in some areas while stalling it in others.

Galaxies

• There are three types of galaxies in our universe:• Spiral• Ellipical• Irregular

Spiral GalaxySpiral galaxies are complex objects and have several components: a disk, a bulge, and a halo. The disk contains gas, dust, and young stars in its spiral arms. The dense bulge in the center of the disk contains mostly old stars and no gas or dust.The four distinguishing characteristics of the spirals are: (a) they have more orderly, rotational motion than random motion (the rotation refers to the disk as a whole and means that the star orbits are closely confined to a narrow range of angles and are fairly circular); (b) they have some or a lot of gas and dust between the stars; (c) this means they can have new star formation occurring in the disk, particularly in the spiral arms; and (d) they have a spiral structure.

We live in a spiral galaxy…The Milky Way.How do we know the Milky Way is a spiral galaxy?

How do the astronomers tell the shape of our galaxy (the Milky Way), even though it is not possible to take a photograph of it because to do that we would have to go away from it?The clues we have to the shape of the Milky Way are:1) When you look toward the galactic center with your eye, you see a long, thin strip. This suggests a disk seen edge-on, rather than a ellipsoid or another shape. We can also detect the bulge at the center. Since we see spiral galaxies which are disks with central bulges, this is a bit of a tipoff.2) When we measure velocities of stars and gas in our galaxy, we see an overall rotational motion greater than random motions. This is another characteristic of a spiral.3) The gas fraction, color, and dust content of our galaxy are spiral-like.

Elliptical Galaxies

Elliptical galaxies are the most abundant type of galaxies found in the universe. However, because of their age and dim qualities, they are frequently outshone by younger, brighter collection of stars.Elliptical galaxies lack the swirling arms of their more well-known siblings, spiral galaxies. Instead, they bear the rounded shape of an ellipse, a stretched-out circle. Some stellar collections are more stretched than others.

Because elliptical galaxies contain older stars and less gas, scientists think that they are nearing the end of the evolution line for galaxies. The universe is a violent place, and collisions between galaxies are frequent — indeed, the Milky Way is due to crash into the Andromeda Galaxy in a few billion years. When two spirals collide, they lose their familiar shape, morphing into the less-structured elliptical galaxies.A supermassive black hole is thought to lie at the center of these ancient galaxies. These gluttonous giants consume gas and dust, and may play a role in the slower growth of elliptical galaxies.Born from collision, elliptical galaxies are more commonly found around clusters and groups of galaxies. They are less frequently spotted in the early universe, which supports  the idea that they evolved from the collisions that came later in the life of a galaxy.

Irregular Galaxy

A galaxy that does not have the clearly defined shape and structure of typical elliptical, lenticular, or spiral galaxies. Irregular galaxies typically contain large amounts of gas and dust, and their stars are often young. They account for only a small percentage of known galaxies. Some irregular galaxies are the result of gravitational interactions or collisions between formerly regular galaxies. Many irregular galaxies orbit larger regular ones; the Magellanic Cloud galaxies orbiting the Milky Way are examples.

Irregular galaxies have no particular shape. They are among the smallest galaxies and are full of gas and dust. Having a lot of gas and dust means that these galaxies have a lot of star formation going on within them. This can make them very bright. 

Contents of Galaxies

1.Nebulas2.Globular clusters3.Open Clusters

Nebulas

A nebula is a cloud of gas and dust in space. Some nebulae (more than one nebula) are regions where new stars are being formed, while others are the remains of dead or dying stars. Nebulae come in many different shapes and sizes. There are four main types of nebulae: planetary nebulae, reflection nebulae, emission nebulae, and absorption nebulae. The word nebula comes from the Latin word for cloud.

. Nebulae are the basic building blocks of the universe. They contain the elements from which stars and solar systems are built. They are also among the most beautiful objects in the universe, glowing with rich colors and swirls of light. Stars inside these clouds of gas cause them to glow with beautiful reds, blues, and greens. These colors are the result of different elements within the nebula. Most nebulae are composed of about 90% hydrogen, 10% helium, and 0.1% heavy elements such as carbon, nitrogen, magnesium, potassium, calcium, iron. These clouds of matter are also quite large. In fact, they are among the largest objects in the galaxy. Many of them are dozens or even hundreds of light-years across.

Globular Clusters

Globular clusters are groups of older stars that looks like a ball. There may be up to one million stars in a globular cluster. They are located in the spherical halo that surrounds spiral galaxies and are also found near giant elliptical galaxies

Open Clusters

Open Clusters are groups of closely grouped stars that usually located along the spiral disk of a galaxy. Newly formed open clusters have many bright blue stars. There may be a few hundred to a few thousand stars in an open cluster.

Quasars…what we know very little

about.Quasars are extremely distant objects in our known universe. They are the furthest objects away from our galaxy that can be seen. Quasars are extremely bright masses of energy and light. The name quasar is actually short for quasi-stellar radio source or quasi-stellar object.Quasars are the brightest objects in our universe, although to see one through a telescope they do not look that bright at all. This is because quasars are so far away. They emit radio waves, x-rays and light waves. Quasars appear as faint red stars to us here on Earth.

A quasar is believed to be a supermassive black hole surrounded by an accretion disk. An accretion disk is a flat, disk-like structure of gas that rapidly spirals around a larger object, like a black hole, a new star, a white dwarf, etc. A quasar gradually attracts this gas and sometimes other stars or or even small galaxies with their superstrong gravity. These objects get sucked into the black hole. When a galaxy, star or gas is absorbed into a quasar in such a way, the result is a massive collision of matter that causes a gigantic explosive output of radiation energy and light. This great burst of energy results in a flare, which is a distinct characteristic of quasars.The light, radiation and radio waves from these galaxies and stars being absorbed into a black hole travel billions of light years through space. When we look at quasars which are 10-15 billion light years away, we are looking 10-15 billion years into the past. Pretty amazing, right?

According to the best estimates of astronomers there are at least one hundred billion galaxies in the observable universe. They've counted the galaxies in a particular region, and multiplied this up to estimate the number for the whole universe.