Star Processes and Formation

The Interstellar Medium (ISM)

Composition of the ISMA. Composition by mass

- 75% H, 23% He, and 2% heavier elements such as C, N, O, etc.

- mostly gas plus a little dust

Space is not Empty!

Examples atoms / cm3

Atmosphere 1019

Vacuum cleaner 8 x 1018

Incandescent light bulb 1015

Ultra high vacuum chamber 106

Interplanetary space 10Interstellar space 0.1 to 1Intergalactic space 10-6

Molecular Clouds

Molecular Clouds

Higher density regions of the ISM, often dark

A giant molecular cloud:- 50 ly to 300 ly diameter- 100,000 to 10,000,000 solar masses- Higher Density than typical ISM

- Temperature = 10 K to 30 K (cool enough for molecules to form)

Supernova explosions (exploding stars) produce shock waves that compress the interstellar medium.

Star Formation

Another source of shock waves may be the birth of very hot stars. When such a star is born, the sudden blast of light, especially ultraviolet radiation, can ionize and drive away nearby gas—forming a shock wave that could compress nearby clouds and trigger further star formation.

Star Formation

Even the collision of two interstellar clouds can produce a shock wave and trigger star formation.

Star Formation

The dominant trigger of star formation in our galaxy may be the spiral pattern itself. As interstellar clouds encounter these spiralarms, the clouds are compressed, and star formation can be triggered.

Star Formation

Once begun, star formation can spread like a grass fire.

Star Formation

• A collapsing cloud of gas does not form a single object. Due to instabilities, the cloud breaks into fragments—producing hundreds to thousands of stars.

• http://www.astro.ex.ac.uk/people/mbate/Animations/

Formation of Stars – Star Clusters

• Star forming regions heat the gas to the point where hydrogen is ionized, producing a typically red color. These regions are known as H II regions or emission nebulae.

Formation of Stars – Emission Nebulae

• Observations of small very dense zones of gas and dust called Bok globules within larger nebulae probably represent the very first stages of star formation. These are also the same as anMCC (molecular cloud complex).

Star Formation and MCC’s

• The initial collapse of the gas in an MCC forms a dense core.

• As more gas falls in, a warm protostar develops—buried deep in the dusty gas cloud that continues to contract.

• Throughout its contraction, the protostar converts its gravitational energy into thermal energy.

• As the internal temperature climbs, the center gets hot enough to begin nuclear reactions, and a star is born!

Formation of Stars- Protostars

• The time a protostar takes to contract from a cool interstellar gas cloud to a main-sequence star depends on its mass.

• The more massive the star, the stronger its gravity and the faster it contracts.

The Formation of Protostars

– The sun took about 30 million years to reach the main sequence.

– In contrast, a 15-solar-mass star can contract in only 160,000 years.

– Conversely, a star of 0.2 solar mass takes 1 billion years to reach the main sequence.

The Formation of Protostars

• Only when the protostar is hot enough to drive away its enveloping cloud of gas and dust does it become easy to observe at wavelengths your eye can see. But before that it may beobserved in infrared wavelengths, which penetrate dust.

Protostars

• Observations of jets coming from hidden protostars show that protostars are often surrounded by disks of gas and dust.

• These jets appear to produce small flickering nebulae called Herbig–Haro objects.

Young Stellar Objects (YSO’s)

• T Tauri stars: protostars surrounded by thick disks of gas and dust that are very bright at infrared wavelengths.

Young Stellar Objects (YSO’s)

Star Formation in the Orion Nebula

Left: The Trapezium (hot young stars). Top Right: Proplyds (dusty sacks). Bottom Right: Protoplanetary disk with protostar.

Baby stars are embedded in thick clouds of gas and dust. At their birth, they begin to emit high amounts of UV radiation and strong superwinds, which help to evaporate the cloud and drive the dust away.

Evaporating Gaseous Globules: EGGs

• Some young star systems are surrounded by dusty rings of proto-solar systems.

Young Stars, Young Planets

Disk around Beta Pictoris. Star is occulted (blocked) for a better view.

• Stars are “born” when the core gets hot enough to begin nuclear fusion.

• When fusion begins, its outward push generates enough pressure to stop gravitational contraction of the forming star and its size stabilizes. This is called hydrostatic equilibrium.

• This balance of fusion vs. gravitational forces keeps the star a stable size until late in its life. During this time, the star is on the Main Sequence of the H-R Diagram.

Star Birth

Hydrostatic Equilibrium

• Some protostars are not massive enough to ever begin nuclear fusion, since they will never achieve high enough temperatures and pressures in their cores.

• These “wanna-be” stars still glow red from light generated due to gravitational contraction. They are known as brown dwarfs, but aren’t really brown!

• How does gravity create heat? When a gas is compressed, it converts some of its kinetic energy to heat.

Brown Dwarfs

Brown Dwarfs

• The sun fuses four hydrogen nuclei to make one helium nucleus. – As one helium nucleus contains 0.7 percent less mass than

four hydrogen nuclei, some mass vanishes in the process. – That mass is converted to energy.– You can figure out how much by using Einstein’s famous

equation, E = mc2.

Basic Fusion Reactions

• The Sun has a voracious appetite and needs 1038 reactions per second, transforming 5 million tons of mass into energy every second, just to replace the energy pouring into space from its surface.

• It might sound as if the Sun is losing mass at a furious rate. However, during its entire 10-billion-year lifetime, the Sun will convert less than 0.07 percent of its mass into energy.

Basic Fusion Reactions

Fusion happens only in the core. The simplest and first reaction is:

4 1H → 4He + energy

– 1H represents a proton, the nucleus of a hydrogen atom.– 4He represents the nucleus of a helium atom.– The superscripts indicate the total number of protons and neutrons in each

nucleus.

Basic Fusion Reactions

Four H atoms becomes one He atom through the proton-proton chain.

Basic Fusion Reactions

A second set of fusion reactions begin later at higher temperatures and pressures. They are known as the C-N-O Cycle.

Advanced Fusion: The C-N-O Cycle

• Stars begin their stable lives fusing hydrogen on the lower edge of the main sequence, known as the Zero-Age Main Sequence (ZAMS)

The Life of a Main Sequence Star

• As stars age, gradual changes in luminosity and surface temperature move them upward and slightly to the right.

• By the time they reach the upper edge of the main sequence, they have exhausted nearly all the hydrogen in their centers.

The Life of a Main Sequence Star

• A star’s main sequence position indicates how much hydrogen has been converted to helium in their cores.

• You can use the position of a star in the band, combined with stellar evolution models, as one way to estimate the star’s age.

The Life of a Main Sequence Star

Life Spans of Main Sequence Stars

The H-R Diagram by Population