Characteristics of Stars

• infer the characteristics of stars based on the characteristics of the sun

• discuss how stars are different from planets and identify some stars using their characteristics

CHARACTERISTICS OF STARS

A star is a massive ball of plasma that emits light throughout the universe. While there is only one star in our solar system, there are billions upon billions of stars throughout our galaxy and exponentially more in the billions of galaxies in the universe. A star can be defined by five basic characteristics: brightness, color, surface temperature, size and mass.

Brightness

Two characteristics define brightness: luminosity and magnitude. Luminosity is the amount of light that a star radiates. The size of the star and its surface temperature determine its luminosity. Apparent magnitude of a star is its perceived brightness, factoring in size and distance, while absolute magnitude is its true brightness irrespective of its distance from earth.

Color

A star's color depends on its surface temperature. Cooler stars tend to be redder in color, while hotter stars have a bluer appearance. Stars in the mid ranges are white or yellow, such as our sun. Stars can also blend colors, such as red-orange stars or blue-white stars.

Surface Temperature

Astronomers measure a star's temperature on the Kelvin scale. Zero degrees on the Kelvin scale is theoretically absolute and is equal to -273.15 degrees Celsius. The coolest, reddest stars are approximately 2,500 K, while the hottest stars can reach 50,000 K. Our sun is about 5,500 K.

Size

Astronomers measure the size of a given star in terms of our own sun's radius. Thus, a star that measure 1 solar radii would be the same size as our sun. The star Rigel, which is much larger than our sun, measures 78 solar radii. A star's size, along with its surface temperature, will determine its luminosity.

Mass

A star's mass is also measured in terms of our own sun, with 1 equal to the size of our sun. For instance, Rigel, which is much larger than our sun, has a mass of 3.5 solar masses. Two stars of a similar size may not necessarily have the same mass, as stars can vary greatly in density.

Seven Main Stages of a Star

Stars come in a variety of masses and the mass determines how radiantly the star will shine and how it dies. Massive stars transform into supernovae, neutron stars and black holes while average stars like the sun, end life as a white dwarf surrounded by a disappearing planetary nebula. All stars, irrespective of their size, follow the same 7 stage cycle, they start as a gas cloud and end as a star remnant.

1. Giant Gas Cloud

A star originates from a large cloud of gas. The temperature in the cloud is low enough for the synthesis of molecules. The Orion cloud complex in the Orion system is an example of a star in this stage of life.

2. Protostar

When the gas particles in the molecular cloud run into each other, heat energy is produced. This results in the formation of a warm clump of molecules referred to as the Protostar. The creation of Protostars can be seen through infrared vision as the Protostars are warmer than other materials in the molecular cloud. Several Protostars can be formed in one cloud, depending on the size of the molecular cloud.

3. T-Tauri Phase

A T-Tauri star begins when materials stop falling into the Protostar and release tremendous amounts of energy. The mean temperature of the Tauri star isn’t enough to support nuclear fusion at its core. The T-Tauri star lasts for about 100 million years, following which it enters the most extended phase of development – the Main sequence phase.

4. Main Sequence

The main sequence phase is the stage in development where the core temperature reaches the point for the fusion to commence. In this process, the protons of hydrogen are converted into atoms of helium. This reaction is exothermic; it gives off more heat than it requires and so the core of a main-sequence star releases a tremendous amount of energy.

5. Red Giant

A star converts hydrogen atoms into helium over its course of life at its core. Eventually, the hydrogen fuel runs out, and the internal reaction stops. Without the reactions occurring at the core, a star contracts inward through gravity causing it to expand. As it expands, the star first becomes a subgiant star and then a red giant. Red giants have cooler surfaces than the main-sequence star, and because of this, they appear red than yellow.

6. The Fusion of Heavier Elements

Helium molecules fuse at the core, as the star expands. The energy of this reaction prevents the core from collapsing. The core shrinks and begins fusing carbon, once the helium fusion ends. This process repeats until iron appears at the core. The iron fusion reaction absorbs energy, which causes the core to collapse. This implosion transforms massive stars into a supernova while smaller stars like the sun contract into white dwarfs.

7. Supernovae and Planetary Nebulae

Most of the star material is blasted away into space, but the core implodes into a neutron star or a singularity known as the black hole. Less massive stars don’t explode, their cores contract instead into a tiny, hot star known as the white dwarf while the outer material drifts away. Stars tinier than the sun, don’t have enough mass to burn with anything but a red glow during their main sequence. These red dwarves are difficult to spot. But, these may be the most common stars that can burn for trillions of years.

Describe the phenomenon that occurs as a star reaches the end of its life.