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Stars

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Table of contents

Black holes

Different types of stars

Dying stars create elements

What is a star?

From the perspective of a person here on Earth, a star is a fixed luminous point in the night sky.

When examined more closely they are revealed to be huge, hot, glowing spheres of gas and plasma, most of which are a million times larger than Earth, powered by nuclear fusion.

The very first stars in the universe were originally made of up hydrogen gas and helium gas.

When stars explode they create heavier elements and huge gas clouds. Over tens of millions of years these gas clouds condense together, under the influence of gravity, to create new stars. These new stars start off as mostly hydrogen and helium, but also have other elements.

Sun animation

Size of the Sun

Sun has a diameter 109 times wider than Earth.

Volume depends on diameter x diameter x diameter

Volume = diameter 3

Sun has volume __________________ times larger than Earth.

Sun compared to Earth and Planets

Image public domain from lesud.com, http://www.lesud.com/lesud-astronomy_pageid81.html

How are stars formed?

Observations, combined with computer models, show that solar systems – including stars – formed from giant molecular clouds. Learn more here Formation of the solar system

How are stars powered?

Stars are powered by nuclear fusion. Click here!

What do stars look like in the nighttime sky?

The Bortle Scale: Demonstrating how light pollution adversely affects our view of the night sky. How many stars are visible on a cloudness night in these environments: inner city, urban-suburban transition, suburban, rural, typical dark-sky site, excellent dark-sky site

Interactive app: Demonstrating how light pollution adversely affects our view of the night sky

What is the structure of a star?

Here we see that stars are not simple, but in fact have structures.

How are stars in balance

Stars exist in a balance.

Gravity tries to compress everything to the center

Thermal radiation pressure, from all the nuclear fusion, pushes everything outwards.

As long as these forces are balanced the star is stable.

Star Hydrostatic equilibrium gravity

Source: Nick Strobel’s Astronomy notes, Interior Structure of Stars

In some stars there is more mass, hence more gravity.

That would tend to make the star denser and hotter.

But in response there is more nuclear fusion, increasing the outward pressure.

Again, a balance exists, although the star may be at a different density and size.

Star Hydrostatic equilibrium gravity 2

Source: Nick Strobel’s Astronomy notes, Interior Structure of Stars

What type of star is our Sun?

The Sun is a G-type, main-sequence star.

This is a Hertzsprung–Russell diagram. Stars of greater luminosity are toward the top. Stars with higher surface temperature are toward the left.

Hertzsprung-Russell Diagram

Credit ESO (European Southern Observatory), eso0728c

The Sun is often informally referred to as a yellow dwarf (although its light is closer to white than yellow.)

About 4% of stars in our galaxy are of this type.

This type of star outshines most other types of stars.

What is the expected fate of our sun?

A G-type main-sequence star will fuse hydrogen for approximately 10 billion years, until it runs out of hydrogen molecules at the center of the star.

When this happens, the star expands to many times its previous size and becomes a red giant, such as Aldebaran.

Eventually the red giant sheds its outer layers of gas, which become a planetary nebula, while the core rapidly cools and contracts into a compact, dense white dwarf.

(from wikipedia)

Comparison_of_planets_and_stars_(sheet_by_sheet)_(Jan_2015_update)

Videos/resources

Short videos for stars/black holes.

Learning Standards

HS-ESS1-1. Use informational text to explain that the life span of the Sun over approximately 10 billion years is a function of nuclear fusion in its core. Communicate that stars, through nuclear fusion over their life cycle, produce elements from helium to iron and release energy that eventually reaches Earth in the form of radiation.

A Framework for K-12 Science Education: Practices, Crosscutting Concepts, and Core Ideas (2012)

Stars’ radiation of visible light and other forms of energy can be measured and studied to develop explanations about the formation, age, and composition of the universe. Stars go through a sequence of developmental stages—they are formed; evolve in size, mass, and brightness; and eventually burn out. Material from earlier stars that exploded as supernovas is recycled to form younger stars and their planetary systems. The sun is a medium-sized star about halfway through its predicted life span of about 10 billion years.

By the end of grade 12. The star called the sun is changing and will burn out over a life span of approximately 10 billion years. The sun is just one of more than 200 billion stars in the Milky Way galaxy, and the Milky Way is just one of hundreds of billions of galaxies in the universe. The study of stars’ light spectra and brightness is used to identify compositional elements of stars, their movements, and their distances from Earth.

ESS1.B  Some objects in the solar system can be seen with the naked eye. Planets in the night sky change positions and are not always visible from Earth as they orbit the sun. Stars appear in patterns called constellations, which can be used for navigation and appear to move together across the sky because of Earth’s rotation.

Benchmarks: American Association for the Advancement of Science

The patterns of stars in the sky stay the same, although they appear to move across the sky nightly, and different stars can be seen in different seasons. 4A/E1

Stars condensed by gravity out of clouds of molecules of the lightest elements until nuclear fusion of the light elements into heavier ones began to occur. Fusion released great amounts of energy over millions of years. 4A/H2cd

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