Star Formation & Main-sequence Stars (Power Point 15)

Student Learning Objectives Describe stellar formation Describe the interstellar medium and its effects on light Compare and contrast types of nebulae Describe the process of star formation Recall the balance that maintains stellar structure

What is the interstellar medium and how is it detected? The interstellar medium (ISM) is gas and dust between the stars. Giant clouds Stellar nurseries The ISM can be detected because it alters light from stars. Interstellar extinction Interstellar reddening Absorption lines Strong emission line (21 cm)

Practice The ISM produces a strong emission line at 21 cm (Radio). What does this tell us about the energy in an ISM cloud? How many stars can a single ISM cloud produce?

Emission Nebula Emission nebulae (HII regions) Virtually no dust grains Associated with young stars The new star excites surrounding gasses. Emission Nebula IC 1396 APOD 2007 December 24

Reflection Nebula Reflection nebulae The dust scatters starlight. Very few dust grains Associated with young stars The dust scatters starlight. Image Credit: NASA

Dark Nebula Dark nebulae May contain embedded objects. many dust grains Dust blocks light May contain embedded objects. APOD 2009 June 15

Practice What is the key aspect that determines the appearance (type) of nebula? In which nebula would you expect to find a star beginning to form? Why does the excited gas of an emission nebula glow red?

Why are interstellar dust grains important? Solid dust grains account for about 1% of ISM. Grains align with magnetic field. Magnetic maps indicate where stars are beginning to form.

The Astrophyiscal Journal, Published 9 September 2009 “A TWO MICRON ALL SKY SURVEY ANALYSIS OF THE STABILITY OF SOUTHERN BOK GLOBULES” The Astrophyiscal Journal, Published 9 September 2009

What is the process of star formation? ISM clouds are essentially stable. Inward Pull   Gravity pulls toward centers of mass Gravity may cause collapse Balanced Outward Push   Magnetic fields maintain shape Rotation and turbulence resist collapse

Cloud Collapse Cloud collapse begins when gravity wins against pressure. Cloud is cold (10K) Outward pressure against gravity is low An event disrupts the gravity-pressure balance

Video by Our Universe Visualized https://www.youtube.com/watch?v=_c7rNfiQdjI Video by Our Universe Visualized

Practice Explain why rotation helps to hold up a cloud against gravity. What are the possible events that could disrupt the gravity-pressure balance?

Image Credit: Gemini Observatory/Artwork by Jon Lomberg Warm Dense Object Bok Globules form first. Embedded in dark nebulae Dense gas regions heated by contraction Pre-fusion masses Bok globules become either a star or a Brown Dwarf. Brown Dwarf < 0.08 Msun Image Credit: Gemini Observatory/Artwork by Jon Lomberg

Proto-star A proto-star is an object that has enough mass to evolve into a star. Bok globule accretes “enough” mass Warm dense core Associated envelope of material Object is heating and rotating Contraction + Accretion  Heat + Rotation

Formation of a solar system https://www.youtube.com/watch?v=E4yirtvUurA Video by Our Universe Visualized

HR Diagram Birth Tracks Star HR Diagram Birth Tracks The star “turns on”. Proto-star enters main-sequence, and is now classified as a star. Image Credit: pics about space

Practice Why does the proto-star get fainter and then grow in brightness? When does a proto-star become a star? What is it that classifies an object as a star?

Image Credit: Hubble Archive, NASA, & ESA https://apod.nasa.gov/apod/ap140204.html Image Credit: Hubble Archive, NASA, & ESA

A star spends 90% of lifetime in the main-sequence stage. How long do stars live? A star spends 90% of lifetime in the main-sequence stage. Mass (Solar Units) Formation (years) Main-Sequence 1 1 Billion 90 Billion 5 5 Million 60 Million 10 0.6 Million 10 Million

Temperature determines the type of nuclear fusion. Low Mass High Mass T = 7 Million K T = 16 Million K PPC CNO 4H  He C + N + O + H  He

Practice Which type of nuclear fusion occurs in our Sun? Which type of nuclear fusion occurs in a Blue Star? What type of nuclear fusion occurs in an M-type Star? Which nuclear fusion process do you expect to produce more energy per second?

What maintains stellar structure? The outward radiation pressure balances the inward force of gravity, at every layer of the star. (maintains shape of star) Hydrostatic Equilibrium Inward Gravity Outward Pressure

Hydrostatic Equilibrium at each layer Image Credit: Astronomynotes.com

Practice At which layer of the star, innermost or outermost, is gravity greatest? Why? At which layer of the star, innermost or outermost, is the outward pressure greatest? Explain. Compare hydrostatic equilibrium in a star to what you have experienced in a swimming pool.