Stars/Evolutions

The Cat's Eye Nebula, a planetary nebula formed by the eruptions of one or more stars at its center.

The star, or star system, at the center of the Cat's Eye Nebula on the right, is a Wolf-Rayet class WC star. An O7 spectral class star may be present as part of the system.

The planetary nebula around the stars has reached an outer diameter of about 25 arcseconds. If it has been expanding at a constant rate of 10 milliarcseconds a year, then HD 164963, the WC star, also designated BD +66 1066, has been evolving at least over the last 25,000 years.

Introduction

How do stars form, how do they shine for such a long period of time, and how do they die?

This lecture seeks to describe the process of a star's evolution.

Stellar evolution can be described as the life and death of a star. A period that theoretically may last from a few hundred million to ten billion years depending on the size of the star. Our Sun is about 4.5 billion years old and should last another 4-6 billion years if the Sun is an average size star.

If you were to pick a star at random there would be 1 in 15 chance of it being similar (same spectral type) as the sun. On the other hand the most likely star you would pick would be a very low mass M-type star which make up about 76% of all stars in our galaxy, the Milky Way.

Other stars with larger masses, about ten times that of our Sun's, have a chance that they may burst and throw out all of their stellar material into the surrounding space leaving behind a super-dense object called a neutron star.

One theory is that throughout a star's life, it consumes its hydrogen through the process of nuclear fusion in its core, creating new and heavier elements until the star has exhausted its hydrogen fuel.

Pre-requisites

Students should have a good grasp of classical physics and calculus.

Evolutionary factors

• Initial Conditions
  • Collapsing Gas Clouds
  • The Virial Theorem
  • Jeans Mass
  • Kelvin-Helmholz Contraction

• Nuclear Fusion in Stars
  • The proton-proton chain

Objectives

Students will acquire an understanding of the following: The stages of the evolution of a star are determined by the size (mass) of the star.

Theoretical stellar evolution

Def. the "process of accumulating change" is called evolution.

Def.

1.a: "any natural luminous body visible in the sky [especially] at night",

1.b: "a self-luminous gaseous celestial body of great mass whose shape is [usually] spheroidal and whose size may be as small as the earth or larger than the earth's orbit".

is called a star.

Here's a theoretical definition:

Def. the "process of accumulating change" of "a self-luminous gaseous celestial body of great mass whose shape is [usually] spheroidal and whose size may be as small as the earth or larger than the earth's orbit" is called stellar evolution.

Photospheres

Main resources: Stars/Photospheres and Photospheres

Def. "[a] visible surface layer of a star, and especially that of a sun" is called a photosphere.

"When we speak of the surface of the Sun, we normally mean the photosphere."

"[T]he photosphere may be thought of as the imaginary surface from which the solar light that we see appears to be emitted. The diameter quoted for the Sun usually refers to the diameter of the photosphere."

The photosphere usually emits visual, or visible, radiation.

Starspots

Main resources: Stars/Spots and Starspots

Doppler maps are of the highly active star BO Mic ('Speedy Mic') at different rotation phases (indicated on top of the maps). Credit: ESO, European Southern Observatory.

"Starspots are equivalent to sunspots but located on other stars. Spots the size of sunspots are very hard to detect since they are too small to cause fluctuations in brightness. Observed starspots are in general much larger than those on the Sun, up to about 30 % of the stellar surface may be covered, corresponding to sizes 100 times greater than those on the Sun."

"The distribution of starspots across the stellar surface varies analogous to the solar case, but differs for different types of stars, e.g., depending on whether the star is a binary or not. The same type of activity cycles that are found for the Sun can be seen for other stars, corresponding to the solar (2 times) 11-year cycle. Some stars have longer cycles, possibly analogous to the Maunder minima for the Sun."

The Sun and other stars have periods of no starspots. As starspots appear to be cyclical but leave no apparent accumulated change, starspots are not participating in stellar evolution.

Understanding

Illustration is of the life-cycle of the Sun. Credit: User:Tablizer.{{free media}}

• Review what you have learned about stars so far.
• Do you fully understand the terms red giant, white dwarf, neutron star, supernova, and black hole? If not, become acquainted with the terms and understand how they relate to the evolution of stars of different sizes.

Before continuing the activity, you should understand the following three facts:

1. A star the size of our sun will burn steadily for 10 billion years, then expand to a red giant, and finally collapse into a white dwarf about the size of Earth.
2. A star three or four times the sun’s mass will burn steadily for a shorter time, then expand into a red giant, and finally collapse, ending up as a neutron star—a super-dense star about the size of a large city.
3. A star 50 times the sun’s mass will burn for an even shorter time and may blow up as a supernova before collapsing and eventually shrinking to infinity, becoming a black hole.

Focus on one of the three types of stars: a star the size of our sun, a star three or four times the sun’s mass, and a star 50 times the sun’s mass.

Draw a set of diagrams for all three types of stars.

Example diagram is centered at top.

Discussion Questions

1. Discuss why some scientists were uneasy about the idea of an expanding universe?
2. Astronomer Wendy Freedman's observations of Cepheid variable stars in another galaxy indicated that the age of the universe is about eight to twelve billion years. Why did her discovery cause such a debate among astronomers? What elements of her discovery still lend themselves to argument?
3. What do scientists learn by observing parts of the universe in other than the visible parts of the spectrum?
4. What materials are believed to compose dark matter, and what can we learn about the universe by examining it?

Evaluation

Evaluations of the diagrams are as follows:

Three points: diagrams carefully prepared; labels clear and correct; diagrams accurately illustrate the star’s stages of evolution

Two points: diagrams adequately prepared; some labels unclear or incorrect; diagrams accurately illustrate the star’s stages of evolution

One point: diagrams carelessly prepared; labels unclear and/or incorrect; diagrams reflect some inaccurate information about the star’s stages of evolution

Students can contribute to the assessment by determining how many diagrams will be required to illustrate the stages of evolution for each type of star.

Extra

• Cepheid variables, supernovae, dark matter, cosmic background radiation, black holes, red shift.

See also

• Stars/Astronomy