Ch. 21 The Birth of Stars The Cone Nebula

Milky Way Photo, showing dark clouds and nebula

Interstellar matter Gas and dust Dust grains about 10-7 m in diameter, about the size of smoke particles Dust causes reddening of the light that passes through it, but NOT redshift. This is due to absorption of the blue components of light (more-so in UV).

FIGURE 12-4 Interstellar Reddening (a) Dust in interstellar space scatters more short-wavelength (blue) light passing through it than longer-wavelength colors. Therefore, stars and other objects seen through interstellar clouds appear redder than they would otherwise. (b) Light from these two nebulae pass through different amounts of interstellar dust and therefore they appear to have different colors. Because NGC 3603 is farther away, it appears a ruddier shade of red than does NGC 3576. (Anglo- Australian Observatory)

Light Reddening due to absorption by dust.

Light from a star has some absorption lines due to the chromosphere.

Additional light reddening is due to absorption by dust.

Reddening in Earth’s Atmosphere colors the sunsets.

FIGURE 12-4 Interstellar Reddening (a) Dust in interstellar space scatters more short-wavelength (blue) light passing through it than longer-wavelength colors. Therefore, stars and other objects seen through interstellar clouds appear redder than they would otherwise. (b) Light from these two nebulae pass through different amounts of interstellar dust and therefore they appear to have different colors. Because NGC 3603 is farther away, it appears a ruddier shade of red than does NGC 3576. (Anglo- Australian Observatory)

Interstellar gas The interstellar gas is very dilute, about ONE atom per cubic centimeter. In some places it is much denser. The distribution of gas is very uneven. It is mostly Hydrogen (90%), Helium (9%), and everything else (1%).

Interstellar Clouds: A Milky Way mosaic shows dark clouds.

A third of the Milky Way Mosaic.

Another third of the Milky Way Mosaic.

Another third of the Milky Way Mosaic.

Galactic Plane, showing several nebulae. blow up this small frame

M20–M8 Region blow up this small frame

M20 (Messier 20) The Trifid Nebula

Trifid nebula, taken with ground-based telescope. Insert shows field of view of Hubble wide-field camera.

Detail of Trifid nebula, showing a pillar of cold molecular gas, & a jet coming out of a hidden star, which is about 0.5 parsec long. Blow up the upper-left corner.

Detail of Trifid nebula, showing a pillar of cold molecular gas, & a jet coming out of a hidden star, which is about 0.5 parsec long. Expand this some.

Detail of Trifid nebula, showing a pillar of cold molecular gas, & a jet coming out of a hidden star, which is about 0.5 parsec long.

Emission Nebulae Ultraviolet light causes hydrogen to glow with a pinkish color. The detail is in false colors. See the following slides for blowups of these.

M8 – The Lagoon Nebula

M8 – The Lagoon Nebula

M8 – The Lagoon Nebula – detail in false color

from the Hubble website: www.hubblesite.org

M16 - The Eagle Nebula

M16 - The Eagle Nebula, in visible light

M16 - The Eagle Nebula, in visible light - close up of pillar region.

M16 - The Eagle Nebula – the pillars in false color, from the Hubble Space Telescope. Blow up this corner, and rotate it. These are sometimes called the “Pillars of Creation”

EGGs – Evaporating Gaseous Globules These can be seen as pillars and egg-like objects.

detail of EGGs: Evaporating Gaseous Globules these can be seen as pillars and egg-like objects

finer detail of these EGGs: Evaporating Gaseous Globules

Some properties of these nebulae: note that these are AVERAGE quantities; the nebulae are actual quite uneven in their density and temperature. Note the huge masses and sizes.

Figure 11.9 Emission Nebula Spectrum

Dark Dust Clouds: Obscuration and Emission more on next slides

Dark Dust Clouds: not just an absence of stars!

Radio Emission reveals the dark dust cloud.

Dark Dust Cloud, seen in visible light.

Dark Dust Cloud revealed in infrared photo

Horsehead Nebula (neck is about 0 Horsehead Nebula (neck is about 0.25 pc across) The pink nebulae are emission nebulae.

Horsehead Nebula (neck is about 0 Horsehead Nebula (neck is about 0.25 pc across) A reflection nebula is seen to the lower left of the horsehead.

Horsehead Nebula (The neck is about 0 Horsehead Nebula (The neck is about 0.25 pc across) A nice example of a dark dust cloud

Hydrogen 21-cm Emission (shortwave radio)

Molecules near M20, visible photo with contour plot of 21 cm radio intensity This cold dark cloud is probably in stage 1 of star formation

Molecular Cloud Complexes in outer portion of the Milky Way galaxy, looking away from the center. This is a false-color image which corresponds to the intensity of the emission from CO gas. The density in these molecular clouds can be a million times the average, or about one million molecules per cubic centimeter.

Star formation – a 7 stage process 1 – an interstellar cloud 2 – shrinking cloud fragments 3 – a fragment is the size of our solar system 4 – protostar center reaches 1,000,000 K 5 – protostar at ~10 solar radius, 4000K surf. 6 – ignition of fusion in core, now a star 7 – reaches main sequence

Atomic Motions are rarely influenced by gravity Atomic Motions are rarely influenced by gravity. They just keep colliding and coming apart. When there are enough in a cloud to equal the mass of the sun, and temperature is about 100 K, the entire cloud can start to shrink due to its own weight, and we get stage 1 of star formation. (The “collapse” of a cloud is probably “triggered” by some event in nearby space.)

Stage 2: Cloud Fragmentation probably occurs Fragments may contain one to several solar masses of molecular gas and dust.

Orion Nebula, Up Close

Orion Nebula is located in the “sword” of several objects hanging below the “belt” of Orion.

Orion Nebula, Up Close

Orion Nebula, A closer look reveals “knots” or EGGs, some of which may contain protostars.

These globules may contain evolving planets as well as the central protostar.

Several disks that may be protoplanetary disks are found after blowing up the Hubble photo.

A protostar can be plotted on the H–R diagram after reaching stage 4 A protostar can be plotted on the H–R diagram after reaching stage 4. It is heated solely due to contraction and is fairly cool, but might be 1000 times as luminous as our Sun, mostly in the infrared part of the spectrum.

Interstellar Cloud Evolution toward a protostar.

Stages of evolution of a star like the Sun

Newborn Star on the H–R Diagram Stage 5 – T Tauri stage – has violent surface activity and may form “jets” Stage 6 – core at 10 million K and finally get fusion Stage 7 – reaches the main sequence

Protostellar Outflow

Protostars in Orion region top: low-mass protostar with solar-system- sized dusty disk bottom: more evolved disk around a protostar

Prestellar Evolutionary Tracks for stars of other masses The minimum mass needed to get nuclear fusion and produce a real star is about 0.08 solar mass, or about 80 times the mass of Jupiter. With less mass all we get are “brown dwarfs”

FIGURE 12-12 Pre–Main-Sequence Evolutionary Tracks This H-R diagram shows evolutionary tracks based on models of seven stars having different masses. The dashed lines indicate the stage reached after the indicated number of years of evolution. The birth line, shown in blue, is the location where each protostar stops accreting matter and becomes a pre–mainsequence star. Note that all tracks terminate on the main sequence at points that agree with the mass-luminosity relation (see Figure 11-14a).

Brown Dwarfs are “failed stars” This one has a mass of about 50 times that of Jupiter. The line is a CCD glitch

We believe that most stars form in clusters from a single large cloud that fragments. An example of an Open Cluster is the Pleiades cluster (M45, a.k.a. “the seven sisters”).

H-R diagram of the Pleiades Open Cluster

Globular Cluster – Omega Centauri

H-R diagram of Globular Cluster Omega Centauri contains no main sequence stars with mass greater than 0.8 solar mass over 10 billion years old.

FIGURE 12-5 A Gas- and Dust-Rich Region of Orion (a) This color-coded radio map of a large section of the sky shows the extent of giant molecular clouds in Orion and Monoceros as seen in the radio part of the spectrum. The intensity of carbon monoxide (CO) emission is displayed by colors in the order of the rainbow, from violet for the weakest to red for the strongest. Black indicates no detectable emission. The locations of four prominent star-forming nebulae are indicated on the star chart overlay. Note that the Orion and Horsehead nebulae are sites of intense CO emission, indicating that stars are forming in these regions. (b, c) A variety of nebulae appear in the sky around Alnitak, also called (zeta) Orionis, the easternmost star in the belt of Orion. To the left of Alnitak is a bright, red emission nebula, called NGC 2024. The glowing gases in emission nebulae are excited by ultraviolet radiation from young, massive stars. Dust grains obscure part of NGC 2024, giving the appearance of black streaks, while the distinctively shaped dust cloud, called the Horsehead Nebula, blocks the light from the background nebula IC 434. The Horsehead is part of a larger complex of dark interstellar matter, seen in the lower left of this image. Above and to the left of the Horsehead Nebula is the reflection nebula NGC 2023, whose dust grains scatter blue light from stars between us and is more effectively than any other color. All of this nebulosity lies about 1600 light-years from Earth, while the star Alnitak is only 815 light-years away from us. NGC refers to the New General Catalog of stars and IC stands for Index Catalogs, two supplements to the NGC. (a: R. Maddalena, M. Morris, J. Moscowitz, and P. Thaddeus; b: Royal Observatory, Edinburgh; c: R. C. Mitchell, Central Washington University, respectively)

Relation between clusters and nebulae: Young Stars in Orion top: visible photo shows the nebula bottom: IR photo shows the stars more clearly, note the four central stars (the Trapezium) see next slides

Young Stars in Orion visible photo shows the nebula

Young Stars in Orion IR photo shows the stars clearly, note the four central stars (the Trapezium)

FIGURE 12-8 The Core of the Rosette Nebula The large, circular Rosette Nebula (NGC 2237) is near one end of a sprawling giant molecular cloud in the constellation Monoceros (the Unicorn). Radiation from young, hot stars has blown gas away from the center of this nebula. Some of this gas has become clumped in Bok globules that appear silhouetted against the glowing background gases. New star formation is taking place within these globules. The entire Rosette Nebula has an angular diameter on the sky nearly 3 times that of the Moon, and it lies some 3000 light-years from Earth. (Anglo-Australian Observatory)

FIGURE 12-6 A Dark Nebula The dark nebula Barnard 86 is located in Sagittarius. It is visible in this photograph simply because it blocks out light from the stars beyond it. The cluster of bluish stars to the left of the dark nebula is a member of a star cluster, called NGC 6520. (Anglo-Australian Observatory)

FIGURE 12-9 Protostar in a Bok Globule (a) This visible light image shows a small dark nebula (equivalently, Bok globule) called L1014 located in the constellation Cygnus. (b) When viewed in the infrared, a protostar is visible within the nebula. (a: Deep Sky Survey; b: NASA/JPL-Caltech/N. Evans, University of Texas at Austin)