The interstellar medium and its impact on star formation, made up of the atoms, molecules, and dust between the Milky Way’s stars, can be studied to learn more about how stars develop and how planets and stars form in connection with that process.
1. What is Interstellar Medium?
The area between stars is known as the interstellar medium and is home to enormous, diffuse dark clouds of gases and tiny solid dust particles. Almost 5% of the mass of the Milky Way galaxy, where the Earth is located, is made up of this fragile stuff in the interstellar medium.
1.1 Components of Interstellar Medium
The main component is hydrogen gas. Moreover, a small amount of helium and minor quantities of formaldehyde, calcium, salt, water, and ammonia have all been found.
Large amounts of dust particles with an unknown composition are also found. The interstellar areas are traversed by primary cosmic rays and magnetic fields.
1.1.1. Major Components
The majority of interstellar stuff is found in low densities, resembling clouds, which occasionally condense to the point where stars can form. These stars, in turn, continue to lose mass, sometimes through minor eruptions and sometimes through supernovae, which are cataclysmic explosions.
In this way, the mass is returned to the interstellar medium where it interacts with material that hasn’t yet given rise to stars.
In the Milky Way Galaxy, this scattered matter is found in the system’s outer regions (the so-called spiral arms), which are also home to several newborn stars and planetary nebulae.
This material is heavily concentrated in a flat area known as the galactic disc.
1.2 Physics Behind Interstellar Medium
Many techniques are used to study the physics of interstellar medium. Up to the middle of the 20th century, almost all knowledge was discovered by using optical telescopes to study the effects of this matter on the light from distant stars.
The interstellar medium has been extensively studied using optical and radio radiation. By gradually fading with distance, the stars in the Galaxy, especially those scattered along the Milky Way, demonstrate the existence of an all-pervasive, general interstellar reddening medium.
Interstellar dust, which obscures interstellar reddening starlight takes place, is the main source of this process. As a result, the light from a star in the plane of the Galaxy that is 6,000 light-years away will look four times fainter than it would if interstellar dust weren’t present.
Generally speaking, the galactic plane from stars is where the electric vectors prefer to be located, while there are some regions from stars where the distribution is more erratic.
As a result, it is possible to draw the direction of the magnetic field in the Milky Way using the polarisation orientations for stars in various regions of the sky.
The typical density of interstellar gas close to the Sun is 1021 gm/cm3, or around one hydrogen atom per cubic centimeter.
2. Why Study Interstellar Medium?
Why is the interstellar medium a subject of study? The intergalactic medium turns out to be some pretty cool stuff!
The universe’s initial composition was primarily composed of hydrogen and a small quantity of helium with minor amounts of lithium, beryllium, and boron, which was not particularly diversified.
The first stars appeared shortly after the first astrophysics of galaxies formed, and they had thrilling but brief lives. Within these gigantic objects, new elements were created by fusing lighter ones to form massive stars by themselves.
These massive stars were thought to have died in the form of enormous explosions called supernovae, where even more elements were created before they were ejected into the chilly interstellar medium.
The following generation of stars (and solar systems) had these new elements, which are essential for life as we know it because the interstellar medium is also where new stars are born.
2.1 Reasons for Astronomers Interested in the Interstellar Medium
- The only sample of outer space that is not from the solar system that we can directly analyze is the nearby interstellar gas;
- Interstellar gas makes up the sun, planets, and all other stars;
- an essential component of the Milky Way and various other galaxies is the interstellar gas cloud;
- The gas’s chemical makeup reveals details of the universe and galaxy developments.
3. Why is the Interstellar Medium Important?
It is surrounded primarily by primordial remains of galaxy formation and star detritus and provides raw components for the future planet and star formation. The study of interstellar media is essential to know how the Universe works and how life cycles form.
4. What is Interstellar Gas?
The temperature of interstellar gas can vary from a few degrees above absolute zero to one million degrees or more, depending on its location. We’ll start our journey across outer space, by learning more about the many circumstances in which gas can be found.
In our galaxy and other galaxies, hydrogen and helium are scattered at various densities between the stars and make up the majority of the gas between the stars. Star formation starts with interstellar gas as its raw material.
Between the massive stars, of our galaxy and other galaxies, the dust is composed of minuscule granules of carbon, iron, and iron-magnesium silicates dispersed at variable high densities and low densities.
In many cases, it is even possible to see the interactions between different interstellar gas concentrations and the background stars to learn more about the kinematics of the gas in various regions of the Galaxy.
5. Molecular Cloud
To their astonishment, astronomers discovered considerably more complicated compounds in interstellar clouds as well, thanks to the development of more advanced instruments for acquiring spectra in the radio and infrared range.
Similar to how atoms leave spectral “fingerprints” in the spectrum of visible blue light, molecules’ internal atom vibrations, and rotations can also do the same in the spectrum of radio and infrared waves.
5.1 All about Wavelengths
The precise wavelengths connected to variations in the rotation and vibration of many common molecules have been revealed to us over the years by research in our labs, providing us with a template of potential lines against which we may now compare our observations of interstellar matter.
Because much of interstellar space is filled with ultraviolet blue light from stars capable of dissociating molecules, the discovery of complex molecules in space came as a surprise (breaking them apart into individual atoms). But looking back, the existence of molecules in the interstellar medium (ISM) is hardly unexpected.
The majority of the other interstellar clouds with hydrogen within them have created the molecule H2 (molecular hydrogen). There are also lesser amounts of additional, more complicated compounds. Large molecular clouds often have high atom concentrations and are substantially denser than interstellar space.
Molecules can form in these deep areas of space where the dark clouds are shielded from starlight. Many complicated chemicals have been discovered in interstellar space due to chemical reactions in gas and on the surface of dust particles and grains.
Although carbon monoxide is very prevalent in many regions of interstellar space, astronomers mostly employ it to examine massive molecular hydrogen clouds. As H2 is extremely cold to emit light, even at radio wavelengths in the majority of huge molecular hydrogen clouds, it is very challenging to see directly.
The fullerenes are called molecular clouds, which have 60 or 70 carbon atoms organized in a cage-like arrangement, and molecular clouds are the biggest compounds yet to be found in interstellar space.
6. What is Emission Nebulae?
Ionized gas clouds called emission nebulae emit light at optical wavelengths on their own, scattering it as the name suggests. They typically have masses between 100 and 10,000 solar masses, and their mass can be dispersed over an area as small as a light year or as large as several hundred light years.
As a result, depending on how compact the nebula is, its densities can range from millions of atoms per cm3 to just a few atoms per cm3.
6.1 The Orion Nebula
One of the most frequent types of emission arises from the ionization of an interstellar gas cloud dominated by neutral hydrogen atoms by nearby O and B-type stars. These hot and bright stars release many high-energy ultraviolet (UV) photons, splitting the cloud of neutral hydrogen atoms into hydrogen nuclei and electrons.
6.1.1. What Happens?
Later on, these combine once again to create neutral hydrogen once more, but this time in an excited ground state. The neutral hydrogen atom releases photons at wavelengths corresponding to the energy disparities between the permitted energy levels of hydrogen when it transitions back to its lowest energy state.
The most significant of these transitions occur at an optical wavelength of 656.3 nm, which is in the red portion of the visible spectrum. It gives emission nebulae their unique red hue blue light also occurs at this wavelength, which is the wavelength of Hα.
Although astronomers usually use the terms HI (pronounced H-one) and HII to denote neutral hydrogen and ionized hydrogen, respectively, this emission nebula is most commonly referred to as HII regions (pronounced H-two region).
Since the O and B stars that ionize the gas survive for a short time and were likely created within the cloud hydrogen gas they are currently irradiating, these nebulae are powerful markers of recent star formation. The Orion Nebula (M42) below Orion’s belt, is one of the most well-known emission nebulae.
6.2 The Planetary Nebulae
Planetary nebulas are another typical category of emission nebula. The primary white dwarf star of these objects is surrounded by a gas cloud that was released as the original star transitioned to the white dwarf phase.
The bluest regions of planetary nebulae are the hottest and represent the regions of greatest excitation because ionizing helium requires a lot more energy than ionizing hydrogen.
7. Interstellar Medium and Its Impact on Star Formation
7.1 What role does the Interstellar Medium play in Star Formation?
Shockwaves traveling in the IMSM interstellar medium can make some areas of the molecules’ molecular cloud very dense and high enough to produce stars. In molecular clouds collapsed by shock waves, they split.
In this regime, the energy intake from radiation, winds, and star explosions can have a significant impact, heating the gas and dust and causing the collapse of giant molecular gas interstellar clouds that break up into stars.
Gravity, turbulence, thermodynamics, magnetic fields, and chemistry are just a few of the many physical factors and physical processes involved in the star formation process.
These processes operate differently from the scale of the entire galaxy to the scale of a single star. As a result, the study of star formation is a discipline that spans many scales and physics.
7.2 What Effects Does the Interstellar Medium Have on Star Formation?
It has two main effects. Grains absorb optical and ultraviolet radiation from stars and re-emit these to a temperature that reflects cold heat from grains. This leads to interstellar extinction.
7.3 Are stars formed from the Interstellar Medium?
Star formation occurs within dense inter-star clouds of dust called molecular clouds. The molecular cloud regions in the Arctic are very cool (temperatures between 20 and 30 K and well below absolute zero – zero degrees). At these levels, gas becomes molecular and translates to atom binding of electrons to the air molecules.
The amount of molecular gas in star-forming galaxies was substantially higher in past cosmic epochs than now.
The star-formation rate against stellar mass main sequence (MS) correlation and, for a given z, the vertical position of a galaxy along it determine the galaxy-integrated depletion period for converting the gas into stars.
The evolution of the cold molecular cloud, gas composition, and star-formation rates of the dominant MS galaxy population is essentially governed by global rates of galaxy gas accretion, which change with cosmic expansion.
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