## A star is born (Part 3 cont.: Evidence of a protostar in a Bok globule)

Bok globules are small interstellar clouds of very cold gas and dust that are nearly totally opaque to visible light as we’ve already seen in a previous post. Although they can be studied with infrared and radio techniques. They were originally discovered as black splotches in front of dense fields of stars, and were even dubbed “holes in the heaven” because they appeared like holes in the stellar background. Bok globules are typically less than 100 solar-masses in size.

A telescope in Chile, has captured in 2013 images of a Bok globule with what astronomers presume is a baby star lighting up an interstellar cloud with jets of gas streaking through deep space at very high speeds, the jet is on the top left corner the Bok globule is the black cloud in the center of the image taken in visible light:

This image from ESO’s New Technology Telescope at the La Silla Observatory in Chile shows the Herbig-Haro object HH 46/47 as jets emerging from a star-forming dark cloud.

This Hubble telescope view of the same region as above but with more details, show a three trillion mile-long jet called HH-47 reveals a very complicated jet pattern that indicates the star (hidden inside a dust cloud near the right bottom edge of the image).

Now, what the astronomers have discovered in 2013, using a radio telescope in Chile this time was quite astonishing. They show the nascent star about 1,400 light-years from Earth, inside our Milky Way, unleashing material at nearly 84,477 mph (144,000 km/h), which then crashes into surrounding gas, causing it to glow.

The glowing object spawned by the newborn star is what scientists call a Herbig-Haro object (That’s why it is also called HH46/47). These new, detailed images showed that the material is streaking out of the star at about 40 kilometers per second (nearly 25 miles per second), One jet appears on the left side of the photo in pink and purple/blue streaming partially toward Earth (this correspond to what we can see in the visible spectrum in the images above), while the orange and green jet on the right-hand-side show a jet pointed away from Earth (this is actually inside the Bok globule and could not be seen i the Hubble picture in the visible spectrum above). The picture is also unusual because the outflow impacts the cloud directly on one side of the young star and escapes out of the cloud on the other:

Astronomers using the Atacama Large Millimeter/sub millimeter Array (ALMA) have obtained a vivid close-up view of material streaming away from a newborn star. By looking at the glow coming from carbon monoxide molecules in an object called Herbig-Haro 46/47 they have discovered that its jets are even more energetic than previously thought. In these observations from ALMA the colors shown represent the motions of the material: the blue parts at the left are a jet approaching the Earth (blue shifted) and the larger jet on the right is receding (red shifted).

Combination of the image above at the beginning of this post in the visible spectrum and the previous infrared and radio spectrum images

Most stars produce powerful outflow jets of matter while they are forming, in a process that is not yet completely understood. It is thought that the jets may help slow the young star’s spin, in the same process that causes a spinning skater to slow down when extending arms outwards. By preventing the star from spinning too fast, the jets may help the star continue its growth by allowing it to continue gathering infalling material. The jets are associated with the presence of an accretion disk that surrounds the young star. Such disks contain the material from which planets and moons could eventually form. Our Sun probably underwent a similar process some 4.5 billion years ago.

Herbig–Haro (HH) objects are small patches of nebulosity associated with newly born stars, and are formed when narrow jets of gas ejected by young stars collide with clouds of gas and dust nearby at speeds of several hundred kilometres per second. Herbig–Haro objects are ubiquitous in star-forming regions, and several are often seen around a single star, aligned with its rotational axis.

These pictures are really breath taking once one understands what nature is doing. I have spent some time trying to put pieces of the puzzle together, but not one single article on this topic has given me enough understanding. I hope that this article will help you grasp the beauty of this phenomenon and make you want to search for more information on for example, star’s accretion disk and outflow jets.

UPDATE 2016-04-17:

In December 2015, the following Hubble image about the topic of this post has been released to the public:
Herbig-Haro Jet HH 24

APOD on HH-24

References:

The Young Star HH 46/47

Pankaj Jain (2015). An Introduction to ASTRONOMY AND ASTROPHYSICS.

Herbig-Haro objects on Wikipedia

ALMA Takes Close Look at Drama of Starbirth

## A star is born (Part 3: The Virial Theorem, Jeans mass and Bok globules)

In this article I will try to show by skipping calculations and focus on the process on how to get to the result, that from a theorem called the Virial theorem, given a molecular cloud composition (mostly hydrogen), known density e.g. n = 4 10^3 cm^-3 and initial temperature. e.g. 50K, it is possible to find out the mass of this collapsing cloud, its radius, and the average luminosity of the protostar as it collapses.

Let’s start with the Virial theorem. Let us consider a system of N particles with masses m_i and position vectors r_i with i = 1,…,N. We define the virial of the system as:

$V = \displaystyle\sum_{i=1}^{N} m_i \vec{r_i} \cdot \dot{\vec r_i}$

Taking the time derivative in dot notation, we obtain:

$\dot V = \displaystyle\sum_{i=1}^{N} m_i \dot{\vec r_i} \cdot \dot{\vec r_i} + \displaystyle\sum_{i=1}^{N} m_i \vec{r_i} \cdot \ddot{\vec r_i} = 2K + \displaystyle\sum_{i=1}^{N} \vec{r_i} \cdot \vec F_i = 2K + U$

Using the facts that Fij=-Fji = m r_dot_dot and that F can be expressed in terms of the gravitational attraction between each particle of the system and that F_ii is zero because particles do not exert any force on itself. We get derivate of V equals 2K + U, two times the kinetic energy plus the potential energy of the system. Now taking the time average of that derivative of V and assuming that it is a periodic system of period T, in that case let’s assume that V(T) is approx V(0) and the time average of the derivative of V is then zero.

$\langle V \rangle = \frac{1}{T} \int\limits_0^T V \mathrm{d}t = \frac{1}{T} [V(T)-V(0)] = 0$

If we skip the time average symbolism and use scalar values for K and U, the Virial theorem is: 2K + U = 0. This means that in equilibrium, the kinetic energy and the gravitational energy balance each other.

An important application of this theorem arises when analyzing the evolution of large gravitationally bound systems, such as clouds of gas and dust in spiral galaxies whose collapse leads to the formation of stars. These clouds normally maintain equilibrium, in which case 2K + U = 0. In a system bound by gravitation, the potential energy of the system is negative (because it is in a “bound state” due to gravitation, an attractive force) and U = – cst. GM^2/R, therefore in 2K = -U, both the lefthand side and the righthand side are positive values.

At some stage they might start collapsing due to some perturbation, such as a nearby supernova explosion. The cloud starts collapsing and the Virial theorem is no longer applicable. In this case, gravitity wins over kinetic energy and 2K is lower than −U. Eventually the heat generated due to collapse leads to an increase in kinetic energy K of the cloud of gas and again returns to equilibrium beacuse If 2K is greater than -U, kinetic energy wins over gravity and the cloud expands.

Given the fact that the gravitational potential energy of a uniform sphere of mass M and radius R is: U = – cstGM^2/R

That the temperature and mean energy of gas particles is (The total energy K+U): cst kT

Jeans Radius for cloud collapse (2K < -U). A cloud with radius R, mass M, and temperature T:

cst k T = cst GM^2/R = cst G rho R^2

Because the mass M of the cloud of gas can be expressed in terms of it radius R knowing the cloud’s density rho (M = cst rho R^3).

$R_{jeans} = \sqrt{\frac{cst kT}{G} \rho}$

$M_{jeans} = (\left(\frac{5kT}{Gm}\right)^{3/2} \sqrt{\frac{3}{4\pi\rho}}) \propto T^{3/2}$

Assuming that the dust/molecular cloud is mostly hydrogen, given the density (rho) and temperature stated above, one can find that the Jean’s radius is approx. between 0,4 and 1 parsec. This means that a cloud will start to collapse if it’s radius is greater than the critical radius or the Jean’s radius, the mass range of the cloud can be calculated too and expressed in terms of solar masses. Using the time it takes for particle to freefall onto the surface of the star (ignoring star’s rotation and other factors), and that gravitational potential energy is converted into radiation. One can find that the luminosity of the cloud can be quite small, which means that the collapsing cloud looks very dark. These regions are sometimes called Bok globules.

References:

Pankaj Jain (2015). An Introduction to ASTRONOMY AND ASTROPHYSICS.

Jeff Hester et al. (2010). 21th Century Astronomy.

## How bright is a star? Introducing apparent and absolute magnitude

In ancient Greece, for approx 2000 years ago, the Greek astronomer Hipparchus was the first to make a catalog of stars according to their brightness. The brightest stars were ‘magnitude 1’, the next brightest were ‘magnitude 2’, etc., down to ‘magnitude 6′, which were the faintest stars Hipparchus and ancient astronomers could see.

The Hipparchus scale is based on the fact that your eyes perceive equal ratios of intensity as equal intervals of brightness. On the quantified magnitude scale, a magnitude interval of 1 unit of magnitude corresponds to a factor of 100^1/5 or approximately 2.512 times the amount in actual intensity. For example, first magnitude stars are about 2.512 times brighter than 2nd magnitude stars, first magnitude stars are 2.512^2 times brighter than 3rd magnitude stars, etc.

Nowadays, CCDs inside digital cameras measure the amount of light coming from stars, and can provide a more precise definition of brightness. Using this scale, modern astronomers now define five magnitudes’ difference as having a brightness ratio of 100. Vega was used as the reference star for the scale. Initially Vega had a magnitude of 0, but more precise instrumentation changed that to 0.3.

The brighter an object appears, the lower the value of its apparent magnitude, with the brightest objects reaching negative values. The thing to remember is that brighter objects have smaller magnitudes than fainter objects and it can have negative values for very bright objects. The magnitude system is based on tradition and therefore seems a little bit odd.

When taking Earth as a reference point, however, the scale of magnitude fails to account for the true differences in brightness between stars. The apparent magnitude, depends on the location of the observer. Different observers will come up with a different measurement, depending on their locations and distance from the star. Stars that are closer to Earth, but fainter, could appear brighter than far more luminous ones that are far away. Therefore, it is useful to establish a convention whereby we can compare two stars on the same footing, without variations in brightness due to differing distances complicating the issue.

The solution was to implement an absolute magnitude scale to provide a reference between stars. To do so, astronomers calculate the brightness of stars as they would appear if it were 32.6 light-years, or 10 parsecs from Earth.

The Sun has for example an apparent magnitude of -26.8 and an absolute magnitude of 4.77. Here is how one can calculate the absolute magnitude of the Sun:

The basic formula relating the apparent (m) and absolute (M} magnitudes is

$M = m + 5 - 5 \log{D}$

where D is the distance to the object in pc.

Sun has m = -26.8 and it is located at 1 A.U. (astronomical unit) from us. 1 A.U. = 1.5 x 10^13 cm = 4.85 x 10-6 pc. Thus, the absolute magnitude of the sun is 4.77.

However, as we have seen in previous posts, since stars can emit light at different wavelengths ranging from high-energy X-rays to low-energy infrared radiation, depending on the type of star, they could be bright in some of these wavelengths and dimmer in others. To address this problem, astronomers must specify which wavelength they are using to make the absolute magnitude measurements. Another limitation is the sensitivity of the instrument (CCDs) used to make the measurement.

To conclude, and make a connection with the previous post on calculating distances to stars, let’s calculate the distance to Tau Ceti (ref) using parallax spectroscopy that we introduced in the previous post on luminosity. Tau Ceti is an interesting nearby star because since December 2012, there has been evidence of possibly five planets orbiting Tau Ceti, with two of these being potentially in the “habitable zone”, which is the zone were you could find among other things liquid water.

Tau Ceti has an apparent magnitude, m = 3.49, and its spectral type is a “G type main sequence star” (ref). From a Hertzsprung-Russell diagram which links spectral type to temperature/luminosity, this indicates an absolute magnitude, M, in the range: +5.0 to +6.5. Using the equation above:

$D = 10 \times \frac{(m-M+5)}{5}$

With M = +5.0, D = 5.00 pc and With M = +6.5, D = 2.50 pc. So the Tau Ceti’s distance to earth is between 2.5 and 5 pc. The Hipparcos/parallax measurements give d = 3.64 pc.

## How bright is a star? Introducing luminosity and its use for calculating distances

If we have a detector on earth of 1 m^2, one can measure something called the “solar constant” or “apparent brightness” which is called “b_sun” ‘b’ as in brightness, a measure of the flux of energy per square meter of detector on earth.

b_sun = 1361 W/m^2 that is the intensity of the sun’s radiation in the vicinity of earth, approximately 100 ordinary incandescent light bulbs of 100 Watts on one square meter. The sun radiates its heat to space, some of it falls on earth the rest is lost to space.

This is to give you an idea of how to measure brightness. Basically, you have to imagine that the detector, instead of being on earth, that it could be placed around the star and then measure the flux of photons that are going through the surface of the detector.

Luminosity is defined as a quantitative measure of the brightness of a star. It is an intrinsic property of the star, it is the power of a star, i.e., the amount of energy (light) that a star emits from its surface in the visible spectrum. There is also bolometric luminosity which I won’t talk about which also take into account radiation outside the visible spectrum.

Luminosity is usually expressed in watts and measured in terms of the luminosity of the sun (L_sun). For example, the sun’s luminosity is 400 trillion trillion watts (energy emitted in joules per second, see the calculation below). One of the closest stars to Earth, Alpha Centauri A, is about 1.3 times as luminous as the sun! It means that if Alpha centauri was our sun, that is if it was physically placed at the same distance from earth, we would receive much more light per square meters here on earth. But how can astronomers measure the luminosity of distant stars? Let’s start by the calculation for the sun.

Apparent Brightness = Luminosity (Energy radiated by a light-source) / Area of the sphere over which the energy is spread

$b = \frac{L}{4 \pi r^{2}}$

The brightness of the light from an object (L constant) is inversely proportional to the square of the distance D from the object. Twice as far means one-fourth as bright.

$b(D) \propto \frac{1}{D^{2}}$

The sun’s luminosity is on earth (at a distance D_sun):

(1) $L = 4 \pi D_{sun}^{2} b_{sun} \approx 4 \times 10^{26} W$

That’s, in power of tens, 400 trillion (10^12) trillion (10^12) watts.

Now, in terms of the flux of energy passing through one square meter of a sphere surrounding the sun, the luminosity can also be expressed as:

(2) L = area of the surface of the sun x flux “F” (W/m^2), so:

$4 \pi D_{sun}^{2} b_{sun} = 4 \pi R_{sun}^{2} \times F$

(3) $F = b_{sun}\frac{D_{sun}^{2}}{R_{sun}^{2}} = 6.29 \times 10^{7} \frac{W}{m^{2}}$

This is the amount of radiated energy by the sun. Every square meter of the sun produces 63 Mega Watts of light, so approximately, 600 thousand 100 watts light bulbs! Given that the sun is huge, that’s a lot of light/energy lost in space.

To measure the luminosity of DISTANT stars (remember that in the previous post, we saw that geometric parallax is only accurate for NEARBY stars), one has to measure the size of the star (or radius) and the star’s temperature. But in most cases neither can be measured directly. To determine a star’s radius, two other metrics are needed: the star’s angular diameter and its distance from Earth, often calculated using parallax. Both can be measured with great accuracy in certain special cases.

So we have 2 categories: stars with luminosity calculated from their geometric parallax and stars with luminosity estimated from their spectrum.

Cat 1: Brightness + distance + inverse square law for dimming allows to calculate intrinsic luminosity (see equation 1)

Cat 2: Scientists use spectroscopic parallax to measure the distance to stars, by assuming that spectra from distant stars of a given type are the same as those from nearby stars of the same type. They use the Hertzsprung-Russell diagram, which gives a place to each star according to the point it has reached in its lifecycle. This method enables scientists to estimate the luminosity of a star that is far away by comparing its spectrum to those of nearer stars.

NOTE: It should first be noted that spectroscopic parallax has nothing to do with geometric parallax. The use of the word parallax here is simply a reference to the goal of finding distances.

However for most stars the angular diameter or parallax, or both, cannot be measure with any certainty. Since the effective temperature is merely a number that represents the temperature of a black body that would reproduce the luminosity, it obviously cannot be measured directly, but it can be estimated from the spectrum.

1. Temperature: A black body radiates power at a rate related to its temperature, that is, the hotter the black body, the greater its power output per unit surface area. An incandescent or filament light bulb is an everyday example. As it gets hotter it gets brighter and emits more energy from its surface. The relationship between power and temperature is not a simple linear one though. The power radiated by a black body per unit surface area is varies with the fourth power of the black body’s effective temperature. The power output, for a perfect black body where sigma is a constant called the Stefan-Boltzmann constant. As a star is not a perfect black body we can approximate this relationship as:

$L \approx \sigma T^{4}$

This relationship helps account for the huge range of stellar luminosities. A small increase in effective temperature can significantly increase the energy emitted per second from each square metre of a star’s surface.

2. Size (radius): If two stars have the same effective temperature but one is larger than the other it has more surface area. The power output per unit surface area is fixed by equation so the star with greater surface area must be intrinsically more luminous than the smaller one.

Assuming stars are spherical then surface area S,where R is the radius of the star, is given by:

$S = 4 \pi R^{2}$

To calculate the total luminosity of a star we can combine these 2 equations:

$L \approx S \sigma T^{4}$

In practice this equation is not used to determine the luminosity of most stars as only a few hundred stars have had their R directly measured. If however, the luminosity of a star can be measured or inferred from other means (eg by spectroscopic comparison) then one can use that equation to determine the radius R of the star.

Let’s get back to distance calulations for Cat 2 stars. Given the equation above to calculate the luminosity of a given star, knowing the sun’s luminosity, the distance to the chosen star (D ‘star’) is given by solving this equation and with D_sun equal to 1 AU:

$\frac{L_{star}}{L_{sun}} = \frac{D_{sun}^{2}}{D_{star}^{2}} \times \frac{b_{star}}{b_{sun}}$

Spectroscopic parallax is only accurate enough to measure stellar distances of up to about 10 kpc. This is because a star has to be sufficiently bright to be able to measure the spectrum, which can be obscured by matter between the star and the observer. Even once the spectrum is measured and the star is classified according to its spectral type there can still be uncertainty in determining its luminosity, and this uncertainty increases as the stellar distance increases. This is because one spectral type can correspond to different types of stars and these will have different luminosities.

Finally, a special type of star called a Cepheid star is very important for measuring distances. With its 41.4-day pulsation period (its luminosity varies in a very regular way), RS Pup is one of the brightest known long-period Cepheids in the Galaxy. But this star also stands out as the only known example of the intimate association of a Cepheid with a dust cloud or reflection nebulae. In the following time-lapse video:

RS Puppis and a reflection nebulae

Observations from the NASA/ESA Hubble Space Telescope over a number of weeks show the variable star RS Puppis and its environment (a large cloud of gas). A phenomenon known as a light echo can be seen around the star, creating the illusion of gas clouds expanding out from RS Puppis. The cloud is not expanding, some of the pulsating light of the star RS Puppis is reflected by the surrounding gas cloud as seen in this video.

The light that travelled from the star to a dust grain and then to the telescope arrives a bit later than the light that comes directly from the star to the telescope. As a consequence, if one measures the brightness of a particular, isolated dust blob in the nebula, one will obtain a brightness curve that has the same shape as the variation of the Cepheid, but shifted in time.

By monitoring the evolution of the brightness of the blobs in the nebula, the astronomers can derive the blobs distance from the star: it is simply the measured delay in time, multiplied by the velocity of light (300 000 km/s). Knowing this distance and the apparent separation on the sky between the star and the blob, one can compute the distance of RS Pup from earth with great accurary. The distance of RS Pup was found to be 6500 light years, plus or minus 90 light years.

To conclude, one last type of star that can be used to measure distances in the universe is a Type 1a supernova. A supernova is an explosion that marks the end of life of certain stars with specific properties. A Type 1a supernova appears in a binary star system in which a white dwarf gather matter from a normal companion star via accretion. Matter streaming from the companion star accumulates on the white dwarf until the white dwarf explodes. While one star is going supernova, the other normal star possibly continues to exist.

Both Cepheid and Type 1a supernova are called “standard candles” because for the former, the pulsation is regular no matter where cepheids are located. For the later, luminosity is assumed to be the same for all Type 1a supernovae, no matter where they are located in the universe. (ref)

APPENDIX:

• A star can be approximated to a black body object (an object which absorbs all light that reaches it). The Stefan-Boltzmann equation gives the relationship between radiated energy from a black body as it heats and its temperature. Flux of radiated energy from the black body = sigma constant times temperature^4. Knowning the flux of the sun, one can calculate its surface temperature, that is 5770K for the photosphere.

## How far from earth is a star? What is Parallax?

This short intro on the concept of parallax is required in order to understand how brightness can be measured for NEARBY stars. The problem is that distances are useful to calculate many astronomical features of stars like luminosity, mass, motion/velocity and size, but distances are very hard to measure. With current technology, given a specific measurement precision, we are basically constrained to stars in our local neighborhood and within the Milky Way.

Literature often use parsecs or ‘pc’ in order to talk about distances. One pc is equal to 3.26 ly. Using parallax is to apply a form of stereoscopic vision, extended by Earth’s orbit, to measure distances to nearby stars. The brain judges distances to objects by comparing the view from the left eye with the view from the right eye. Similarly, if you use the changing perspective of Earth through the year, you can approximate the distances to nearby stars. For example, one can take one picture of a night sky region today and then, wait 6 months and take one picture again of the same region. One can also use two telescope at opposite sides of the earth and observe the same region of the night sky. After comparing the two pictures, some stars will have shifted positions, this is called the ‘parallax’.

In the figure above, with ‘AB’ being the earth diameter and ‘Parallax’ being 2 times the angular parallax, and ‘d’ the distance to the object from the earth center. The angular parallax (p) is equal to and for small angles:

$\sin p \sim p= \frac{AB}{2} / d$

Instead of taking two points on earth, if you take the measurements during summer and winter on the earth orbit. With 1 AU/astronomical unit being the mean distance earth-sun: AB = 2 AU, so the equation becomes:

$d = \frac{1}{p}$

The units are distance in pc (3.26 ly) and p in arcsec (1/3600 degree). The angular parallax (p) is inversely equal to the distance (d) to the star. Therefore, this method does not work in practice for stars that are very far away.

Experimentally, earth-based telescopes can only achieve a parallax measurement of minimum 0.01 arcsec due to the effects of earth’s atmosphere and only 100 stars are within this range of detection from the earth. One satellite/space-based telescope (Hipparcos) has been used to measure the distance to many more nearby stars. Hipparcos stands for High precision parallax collecting satellite and also a reference to the ancient Greek astronomer Hipparchus, the catalogue it produced includes parallax measurements for 100000 stars, with an accuracy of about 0.002 arcsec.

The follow up mission Gaia, is an ESA mission to survey one stars in our galaxy and local galactic neighborhood, in order to build the most precise 3D map of the Milky Way and answer questions about its origin and evolution. The mission’s primary scientific product will be a catalog with the positions, motions, brightness, and colors of the surveyed stars. The nature of the Gaia mission leads to the acquisition of an enormous quantity of data, and the data-processing challenge is a huge: the mission states that about 20 million stars will be measured with a distance precision of 1% and about 200 million will be measured to better than 10%. Distances accurate to 10% will be achieved as far away as the galactic center, 30000 ly away.

To end this post, it is important to stress that parallax is not the only way to measure the distance of stars from earth. There are actually 3 other methods: color/spectroscopy, variable stars and supernovas. The former method can be applied to any stars but it is not very accurate due to matter from the ISM interfering with the measurements. The latter two methods are only applicable to few stars which have specific properties. In the next posts I will introduce these other methods.

## A star is born (Part 2: H-alpha emission nebula)

In this post I will write about a special type of light produced by hydrogen atoms which is extremely important for astronomers in quest of finding star forming regions and newly born stars. By observing the star formation process at different stages in the universe, astronomers can understand how stars are born and how they change from one stage to another.

H-alpha emission is produced by an excited hydrogen atom in which an electron jumps from the n=3 energy level down to n=2 (see picture below on Balmer series source: Balmer series). H-alpha light is visible in the red part of the electromagnetic spectrum (H-alpha emission has a wavelength of 656.281 nm), and is the easiest way for astronomers to trace the ionized hydrogen content of gas clouds. Here is a visualization of Bohr’s model of the Hydrogen atom, n >=2 represents different level quantified/energized electron states while n=1 is called the “ground” state:

Since it takes nearly as much energy to excite the hydrogen atom’s electron from n = 1 (or the “ground state”) to n = 3 as it does to ionize the hydrogen atom (to strip it from its electron), the probability of the electron being excited to n = 3 without being removed from the atom is very small. Instead, after being ionized, the electron and proton recombine to form a new hydrogen atom. In the new atom, the electron may begin in any energy level, and subsequently cascades to the ground state (n = 1), emitting photons with each transition. Approximately half the time, this cascade will include the n = 3 to n = 2 transition and the atom will emit H-alpha light. Therefore, the H-alpha line always occurs where hydrogen is being ionized.

So, in the image below, warm (about 8000 K) interstellar gas glows in H-alpha. H-alpha emission from much of the northern sky, reveals how complex the structure of the ISM is. The faint diffuse emission in the picture comes mostly from warm, ionized hydrogen gas. The especially bright spots, on the other hand, are quite different, these are regions where intense ultraviolet radiation from massive, hot, luminous stars is able to ionize even relatively dense clouds. These bright regions are called H-II (“H-two”) regions, signifying that they are made up of the second, or ionized, form of hydrogen (The first form of hydrogen is the neutral hydrogen atom or simply H-I). H-II regions are the fingerprints of star forming regions.

H-II regions have extremely diverse shapes, because the distribution of the stars and gas inside them is irregular. They often appear clumpy and filamentary, sometimes showing bizarre shapes such as the Horsehead nebula here below. H-II regions may give birth to thousands of stars over a period of several million years. In the end, supernova explosions and strong stellar winds from the most massive stars in the resulting star cluster will disperse the gases of the H-II region, leaving behind a cluster of birthed stars.

The Horsehead nebula, is a dark molecular cloud, roughly 1,500 light years distant, is visible only because its obscuring dust is silhouetted against another, brighter nebula. The prominent horse head portion of the nebula is really just part of a larger cloud of dust. This picture show that the dark clouds in visible light of the Horsehead (left) is a H-II region because when astronomers looked at it in the far infra red (right) they see intense radiation in regions that are opaque in visible light.

## A star is born (Part 1: The Inter-Stellar Medium and dust clouds/nebulae)

In this series of post, I will try to understand how a star is created until it reaches the so-called “main sequence”. Another series of post will look at what is happening after the star leaves the main sequence.

In our galaxy, the Milky Way, the space between the stars is called the ISM or Inter-Stellar Medium. Note that, astronomers also talk about inter-galactic space outside our galaxy and between galaxies and inter-planetary space in the solar system between planets around the sun.

In this post, I mainly write about our local neighborhood of stars and the ISM that fills the volume of our own galaxy and I do not really look at what creates the ISM, I just assume that it is there.

Our Sun formed from the ISM, so it is not surprising that the chemical composition of the ISM in our region of the Milky Way is similar to the chemical composition of the Sun that is mainly hydrogen and helium.

In the ISM, about 90% of the atomic nuclei are hydrogen, and the remaining 10% are almost all helium. More massive elements together account for only 0.1% of the atomic nuclei, or about 2% of the total mass in the ISM. 99% of the matter in the ISM is in gaseous form, consisting of individual atoms or molecules moving freely about, as do the molecules in the air on earth.

Only 1% of the ISM matter is in solid form (solid grains/conglomerates of many molecules and atoms) also called interstellar “dust” but this type of dust has nothing to do with the dust you’ll find at home because it is much smaller. Sometimes this dust creates giant dust clouds or nebulae. In these clouds/nebulae, the dust can be C-based or “graphite”, Si-based or “silicate”, iron, water ice, etc. The analogy with the air and dust on earth ends here because the density of these cloud is on a quite different scale than what you would expect to find on earth.

In a volume of one cubic centimeter of air on earth you have about 10^19 molecules, a good vacuum pump can bring this density down to 10^10 molecules per cubic cm. By comparison, the ISM has in average a density of 1 ATOM per CUBIC cm (remember that one atom is small about 10^-8 cm) !!!!

Giant dust clouds (150-250 ly) within the ISM can have densities as high as 10 10^6 molecules per cubic centimeter. In these clouds the dust is extremely effective at blocking visible light. Because the average size of dust grain is about 10^-6m similar to the wavelength of visible light and therefore, can interfere with visible light. So, dust clouds/nebulae are opaque to visible light. To illustrate this, the picture below shows the Milky Way at different wavelengths. As we can see, gas and dust clouds, which are opaque and dark in visible light (c), are also glowing brilliantly in infrared radiation (b), the explanation for why this intense glow in the infrared is given below.