27.1 The Sun

The Sun is is a giant, hot ball of gas held together by gravity.

The Sun is medium-sized compared with other stars in the universe.

Approximately 1 million planet Earths could fit inside the Sun!

27.1 The Sun

Gravity squeezes the density of a star so tightly in the core that the electrons are stripped away and the bare nuclei of atoms almost touch each other.

Nuclear fusion occurs.

27.1 Anatomy of the sun The apparent

surface of the Sun that we can see from a distance is called the photosphere, which means “sphere of light.”

Just above it is the chromosphere.

This is a very hot layer of plasma, a high-energy state of matter.

27.1 Anatomy of the sun

The corona is the outermost layer of the sun’s atmosphere, extending millions of kilometers beyond the sun.

Sunspots are areas of gas that are cooler than the gases around them.

27.1 Features of the sun

Occasionally, large “loops” of gas called prominences can be seen jumping up from groups of sunspots.

27.1 Features of the sun

Solar wind is an electrically charged mixture of protons and electrons that cause magnetic storms.

Auroras, called the northern lights, occur when layers of our atmosphere are energized by solar winds.

27.1 Solar energy

Solar energy is a term that refers to radiant energy from the Sun.

The radiant energy of the Sun reaches Earth in the form of electromagnetic waves.

We can use solar energy to heat buildings and generate electricity.

27.1 Solar energy

Buildings that use passive solar heating are designed to trap sunlight.

Glass traps warm air, causing a “greenhouse effect.”

27.1 Solar energy

Photovoltaic (or PV) cells are devices that convert sunlight directly into electricity.

Solar cells are found on calculators, watches, or certain outdoor light fixtures.

27.1 More about the Sun’s energy In 1905, Albert Einstein proposed that

matter can be converted into energy.

His famous equation shows how huge amounts of energy can be created from a smaller mass.

27.1 More about the Sun’s energy The amount of this

energy from the Sun that reaches the outer edge of Earth’s atmosphere is known as the solar constant.

The accepted value is 1,386 watts per square meter (W/m2), or about thirteen 100-watt light bulbs per square meter of surface.

27.2 The size of stars Stars come in a

range of sizes and masses.

Our Sun is a medium-sized star.

The largest stars, giant stars have a mass of about 60 times the mass of the Sun.

27.2 The size of stars

There are two types of giant stars.

Red giants are cooler than white stars.

Blue giant stars are hot and much more massive than our sun.

27.2 The size of stars

Stars that are smaller than the sun come in two main categories, dwarfs and neutron stars.

Sirius, the Dog Star, is the largest known white dwarf.

27.2 Temperature and color

If you look closely at the stars on a clear night, you might see a slight reddish or bluish tint to some stars.

This is because their surface temperatures are different.

27.2 Temperature and color

The color of light is related to its energy.

White light is a mixture of all colors at equal brightness.

27.2 Brightness and luminosity

Brightness, also called intensity, describes the amount of light energy per second falling on a surface.

27.2 Brightness and luminosity

Luminosity is the total amount of light given off by a star in all directions.

Luminosity is a fundamental property of a star whereas brightness depends on both luminosity and distance.

27.2 Temperature and luminosity In the early 1900s, Danish

astronomer Ejnar Hertzsprung and American astronomer Henry Russell developed an important tool for studying stars.

Their graph showed luminosity on the y axis…

…and surface temperature on the x axis

27.2 Temperature and luminosity H-R diagrams are useful because they

help astronomers categorize stars into groups: Main sequence stars, like the Sun, are in a

very stable part of their life cycle. White dwarfs are hot and dim and cannot be

seen without a telescope. Red giants are cool and bright and some can

be seen without a telescope. Can you locate blue giants on the H-R

diagram?

27.3 The life cycle of stars

A star, regardless of its size, begins its life inside a huge cloud of gas (mostly hydrogen) and dust called a nebula.

The Eagle Nebula is the birthplace of many stars.

27.3 The life cycle of stars

A protostar is the earliest stage in the life cycle of a star.

The Orion Nebula was the birthplace of these protostars.

27.3 The life cycle of stars A star is born when temperature and

pressure at its center become great enough to start nuclear fusion.

Once nuclear fusion begins, a star is in the main sequence stage of its life cycle.

27.3 The life cycle of stars

The time a star stays on the main sequence depends on the star’s mass.

High-mass stars burn brighter, and hotter, using up their hydrogen faster than low-mass stars.

27.2 The old age of Sun-like stars

With no more energy flowing outward, nothing prevents gravity from crushing the matter in the core together.

When hydrogen fusion stops, the core glows brightly and is called a white dwarf.

27.2 The old age of Sun-like stars

A planetary nebula forms when a star blows off its outer layers leaving its bare core exposed as white dwarf.

Planetary nebulae are one of nature’s ways of recycling the matter in old stars and distributing new elements.

27.3 Supernovae and synthesis of the elements

Scientists believe the early universe was mostly hydrogen, helium and a trace of lithium.

Heavier elements are created by nuclear fusion inside the cores of stars.

Nuclear fusion reactions are exothermic, releasing energy only up to iron.

When the core of the star contains mostly iron, nuclear fusion stops.

27.3 Supernovae and synthesis of the elements

If a star’s iron core reaches 1.4 times the mass of the Sun, gravity becomes strong enough to combine electrons and protons into neutrons.

During this brief period, heavier elements such as gold and uranium are created, as atomic nuclei are smashed together.

The core of the star collapses and the result is a spectacular explosion called a supernova.

27.3 Supernovae and synthesis of the elements

The Crab nebula is the remains of a supernova.

Chinese astronomers recorded it’s demise 1054 AD.

27.3 Examining light from stars

Spectroscopy is a tool of astronomy in which the light produced by a star or other object (called its spectrum) is analyzed.

27.3 Analyzing light from stars A spectrometer splits light into a

spectrum of colors and displays lines of different colors along a scale.

Each element has its own unique pattern of spectral lines.

27.2 Analyzing light from stars

In 1861, Sir William Huggins used spectroscopy to determine that the Sun and the stars are made mostly of hydrogen.

27.2 Analyzing light from stars

A few years later, Sir Joseph Norman Lockyer observed a line at the exact wavelength of 587.6 nm.

He concluded that this must be an undiscovered element and named it helium, after the Greek name for the Sun, Helios.