Chapter 21- StarsChapter 21- Stars

Why will (or won’t) our sun Why will (or won’t) our sun become a black hole?become a black hole?

I. Formation of stars

A. Stars form in cold, dark clouds of gas and dust (nebulae), covering billions of kilometers

B. Requirements

1. Particles within these clouds must be slow moving so that gravity can overcome the internal pressure of the particles and form clumps

2. Clouds must be reasonably dense with hydrogen and helium

C. Process

1. As the cloud shrinks, it breaks up into smaller fragments known as protostars. Hundreds of protostars can be produced this way.

2. A protostar is not hot enough to produce nuclear reactions yet, but as the process of collapse (accumulation of more particles) continues and the protostar becomes more and more dense, the temperature increases to approximately 10 million degrees Kelvin.

3. When the temperature increases sufficiently, nuclear fusion can begin, forming heavier elements

4. Millions of years later, a star is born

II. Life Cycle of Stars

A. Protostars – a hot contracting cloud of dust and gases in a nebula; what happens next depends on how much mass the star began with – may become a medium-sized star, or a massive star

B. Medium-sized stars – time of life cycle depends on the mass of the star when it first formed; the smaller the starting mass the longer it will live (a few billion to 100 billion years)

1. Red giants

a. Hydrogen in a new star’s core is changed to helium by the process of nuclear fusion

b. The helium in the core begins to shrink, and the core heats up again

c. Energy released by the heating of the helium core causes the outer hydrogen shell of the star to expand greatly

d. As the outer shell expands, it cools and its color reddens; at this point the star is considered a red giant or supergiant

e. As the red giant ages, it continues to “burn” the hydrogen gas in its shell, and the helium core continues to get hotter and hotter

f. At about 200,000,000 degrees Celsius, the helium atoms in the core fuse together to form carbon atoms and the last of the hydrogen gas surrounding the red giant begins to drift away

g. This drifting gas forms a shell around the central core of the star; this shell is called a planetary nebula

2. White dwarfs

a. At some point in a red giant’s life, the last of the helium atoms in the core are fused into carbon atoms and the star begins to die

b. Without nuclear fusion taking place in the core, the star slowly cools and fades

c. Gravity causes the last of the star’s matter to collapse inward; matter is squeezed into an extremely dense white dwarf that still shines with a hot, white light

d. At some point, the last of the white dwarf’s energy is gone and it becomes a dead star

It's the burned out corpse of a star named BPM 37093 only about 50 lightyears away from Earth in the region of the sky we refer to as the constellation Centaurus.

The white dwarf star is a chunk of crystallized carbon that weighs 5 million trillion trillion pounds. That would equal a diamond of 10 billion trillion trillion carats

C. Massive stars

1. Difference in development of medium-sized and massive stars -

a. Massive stars start off like medium-sized stars, continuing on the same life cycle path until they become red giants or supergiants

b. In a massive star, the helium in the core turns into carbon, but all carbon atoms are pulled together by gravity

c. The core is squeezed so tightly that the heat given off reaches 600,000,000 degrees Celsius, and the carbon atoms begin to fuse together to form new and heavier elements such as oxygen and nitrogen, and eventually even iron

2. Supernovas

a. Nuclear fusion stops, leaving a main core of mostly iron

b. Iron atoms begin to absorb energy

c. This energy is released in a tremendous explosion called a supernova, reaching temperatures up to 1 billion degrees Celsius

d. At these high temperatures, iron atoms in the core fuse to form new elements which explode into space

e. The resulting clouds of dust and gases forms a new nebula

f. The remaining core of the star will become either a neutron star or a black hole, depending on its starting mass

3. Neutron stars

a. An extremely dense star that began 1.5 – 4 times as massive as the sun

b. Neutron stars spin very rapidly and gives off energy as it spins; this energy is given off in narrow beams called “pulsars”

4. Black holes

a. Results from a star that began with 10 or more times the mass of the sun

Most massive star found, 114 times mass of Sun. 20,000 light years away in southern milky way galaxy.

b. Occurs when a massive core is swallowed up by its own gravity which becomes so strong that not even light can escape

c. Black holes swallow cosmic matter and energy, which are probably squeezed out of existence within the black hole

III. Characteristics of stars

A. Composition

1. Determined by a spectroscope; break up white light into the individual wavelengths

2. Elements:

a. Hydrogen - makes up 60-80% of the total mass

b. Helium – 2nd most common element

c. Other elements such as oxygen, neon, francium, cesium, carbon, and nitrogen total less than 4% of the star’s mass

B. Temperature

1. Surface temperature is determined by observing their color – red is coolest and blue is hottest; surface temperature is much cooler than the star’s core

2. Temperature range:

a. Hottest stars can reach 50,000 degrees Celsius

b. Coolest stars are about 3 thousand degrees Celsius

C. Brightness

1. Depends on the size, surface temperature, and distance from Earth; usually constant, but “variable” stars may vary in brightness

2. Magnitude:

a. Apparent magnitude – the brightness of a star as it appears from Earth

b. Absolute magnitude – the amount of light a star actually gives off

Apparent magnitude

Absolute magnitude

3. Nuclear fusion in the core of a star

causes hydrogen atoms to fuse and

form helium atoms, releasing heat and

light energy and causing stars to shine

Nuclear fusion is the source of energy

for stars.

IV. How do we learn about stars?

A. Hertzsprung-Russell Diagram

1. The surface temperature of stars is plotted along the horizontal axis and the absolute magnitude along the vertical axis

2. Main-sequence stars fall in an area from the upper left corner to the lower right corner and make up more than 90% of the stars in the sky

3. The other 10% of stars are no longer in the main sequence because they have changed as they have aged

a. Above main sequence – red giants and supergiants

b. Below main sequence – white dwarfs

B. Measuring Star Distance

1. Parallax – the apparent change in the position of the star in the sky due to the change in the Earth’s position

a. Apparent position of the star in June and December is noted

b. A line is then drawn between the Earth’s position in these months and the center of the sun, this will become the base of a triangle whose length has already been carefully measured by astronomers

c. A diagonal line is drawn from each end of the base line to the apparent position of the star in June and December

d. The tip of the triangle that has been formed is the true position of the star; a vertical line is drawn from the true position to the base of the triangle, representing the actual distance to the star

2. The distance to stars more than 100 light years away is measured by a complex mathematical formula using the brightness of the star

3. Spectroscopes may be used to measure the distance to galaxies by measuring the amount of red shift in a galaxy’s spectrum

kittykitty

V. A Special Star: Our Sun

A. Our sun is a medium-sized, middle-aged, yellow star about 4.6 billion years old

B. Layers of the sun

1. Corona – the outermost layer of the sun’s atmosphere; temperatures up to 1,700,000 degrees Celsius

2. Chromosphere – the middle layer of

the sun’s atmosphere; temperatures

average 27,800 degrees Celsius

3. Photosphere – the innermost layer

of the sun’s atmosphere; temperatures

usually do not exceed 6000 degrees

Celsius

4. Core – center of the sun; temperatures up to 15 million degrees Celsius; where nuclear fusion takes place

C. The Active Sun

1. Violent storms such as “prominences” may occur on the surface; appear as bright arches or loops of gas

2. Another type of storm called a “solar flare” shows up as a bright burst of light on the sun’s surface

3. A “solar wind” is a continuous stream of high-energy particles released into space in all directions

4. Storms in the lower atmosphere of the sun cause dark areas on the surface of the sun called “sunspots”; they appear dark because they are cooler than the rest of the sun’s surface

a. Periods of very active sunspot activity occur every 10-11 years

b. Because sunspots move across the sun’s surface, astronomers conclude that the gases in the sun spin or rotate on an axis