# Astrophysics

## What to expect

Astrophysics (according to NASA) is the study of how the universe began, works and has evolved, and the search for life on planets around other stars. It's an ever-changing field as human manipulation of technology improves year on year. On 11th April 2019, for example, the first ever photograph of the event horizon of a black hole at the centre of a galaxy was captured.

The IB Physics course contains three key sections at Standard Level:

• Stellar quantities (2 pages) - distinguishing between celestial bodies and defining units of length and brightness to cope with the vast magnitudes at play among the cosmos
• Stellar characteristics and stellar evolution (4 pages) - categorising stars by examining the light they emit and using this information to diagnose a star's position in its lifecycle
• Cosmology (2 pages) - using obervational evidence to make predictions about the past and future of the universe

The Advanced Higher Level topics are stellar processes and further cosmology.

## Key questions

#### How do you find the luminosity of a star?

The luminosity of a star is equivalent to its power, the energy released in one second. Find out more.

#### Is a star a perfect black body?

A star is a good approximation of a perfect black body, an object that emits all wavelengths of electromagnetic radiation. The wavelength of greatest intensity is determined by Wien's displacement law, which states that peak wavelength is inversely proportional to the absolute temperature of the body. Find out more.

#### What are the features of a Hertzsprung-Russell diagram?

A Hertzsprung-Russell (HR) diagram plots stars according to their luminosity (vertical axis) and surface temperature (horizontal axis). The temperature axis is absolute (K), decreasing from left to right and has a non-linear scale. The luminosity axis is logarithmic. The diagonal lines represent radius. Find out more.

#### What is a standard candle?

Some stars act as standard candles. Like a standard solution of known concentration in Chemistry, a standard candle is a star of known luminosity. Find out more.

#### What are the stages of a low mass star?

A star enters the main sequence, as its protostar rises in temperature and commences fusion. It moves from the main sequence to the giant stage, with a reduction in temperature due to the increase in radius and an increase in luminosity associated with helium fusion. At the end of the red giant stage there is a shedding of the planetary nebula as the star expands for the last time. The cooling of a white dwarf corresponds with its decline in luminosity due to the absence of fusion. Find out more.

#### Is there still heat from the Big Bang?

Gazing out into the sky with a radio telescope reveals that the universe is far more homogenous than we might think. A near constant cosmic microwave background (CMB) radiation can be detected in all directions. Scientists had previously speculated that the universe should contain some electromagnetic radiation left over from the Big Bang. At the beginning of the universe this would have been high energy, high frequency gamma radiation. Over time, cooling and expansion of the universe have caused this radiation to increase in wavelength to become the microwave frequency we observe today. Find out more.

#### What is the future of our universe?

If the density of the universe exceeds the critical density, the universe will stop expanding and contract. We call this a closed universe. This would lead to a 'big crunch'! If the density of the universe is equal to the critical density, the rate of expansion of the universe would tend towards zero as time tends towards infinity. The universe is considered flat. If the density of the universe is less than the critical density, the universe will expand forever; an open universe. Find out more.

• #### Objects in the universe

Objects in the universe, including galaxies, planetary systems and stars.

• #### Stellar quantities

The distances between objects in the universe and their brightness.

• #### Spectra

We can perform detective work on stars using their spectrum to determine temperature and chemical composition.

• #### Stellar characteristics

Stars can be displayed on a diagram and categorized in a number of ways.

• #### Cepheid variable stars

Cepheid variable stars, a pulsing star following the red giant stage, act as standard candles.

• #### Stellar evolution

The evolution of a star can be shown on the HR diagram.

• #### Cosmology

Scientists theorize that the universe commenced with a 'Hot Big Bang' of energy and matter. This theory is supported by evidence.

• #### Age of the universe

We can calculate an estimate for the age of the universe based on the velocity at which galaxies are receding from our observations.

• #### AHL Stellar processes

Having established the characteristics and evolution of different types of star, here we will dig a little deeper. Under what circumstances will a nebula collapse? What energy changes are happening during fusion beyond the main sequence? What happens to ma

• #### AHL Further cosmology

In the Cosmology and Age of the universe SL pages we considered cosmological observations and how these act as evidence for the origin and evolution of the universe in the past. Now we will instead look forwards to the future of the universe.