Only 1% of the mass of a molecular cloud is in the form of dust. Molecular clouds consist mainly of gas and dust but can contain stars as well. The material within the clouds is compressed as the cloud collapses, resulting in the creation of stars.
What is a molecular cloud?
A molecular cloud, also called a dark nebula, is an interstellar clump or cloud that is opaque due to its internal dust grains. Such dark clouds have no clearly defined outer boundaries and sometimes seem to have convoluted serpentine shapes because of turbulence. The density of the clumps may reach up to 105 H2 molecules per cubic centimeter or more.
Through the naked eye, the biggest molecular clouds can be seen as dark spots against the Milky Way Galaxy. The so-called enormous molecular clouds, the largest nebulae of this kind, are a million times more massive than the Sun. They are around 150 light-years across, comprise a large portion of the interstellar medium’s mass, and have an average density of 100 to 300 molecules per cubic centimeter with an interior temperature of just 7 to 15 K.
How is the molecular cloud formed?
Molecular clouds can be formed in limited regions where the mean magnetic field is aligned with the direction of shock waves from supernovas, or in regions where a huge number of shock wave sweepings happen. Therefore, molecular clouds can only be seen in limited regions in shells.
What happens to the rotation of a molecular cloud as it collapses to form a star?
Within the regions of molecular clouds, a significant event takes place: the formation of stars from the gravitational collapse of dense clumps in the nebula. Each system has a rotary motion that is a result of the original motions of the material that is falling into it. Because of this spinning motion, the collapsing cloud flattens as it contracts. At some point, the majority of its mass is in a spinning condensation close to the center of a “protostar” that will eventually become one or more stars with close orbits. Surrounding the protostar is a rotating disk that is larger than the solar system which collapses into “protoplanets” and comets.
The evidence for these hypotheses comes from observations of molecular clouds in extremely long-wavelength infrared light. The class of T Tauri variables, named after their prototype star in the constellation Taurus, is one of the brightest infrared sources that is connected to dark dust clouds. The T Tauri stars are always located in or close to molecular clouds and are known to be very young stars. Due to the warm dust clouds that are heated by the T Tauri star to a temperature of a few hundred kelvins, they are frequently also strong producers of infrared radiation.
Which trigger could start the gravitational contraction of a molecular cloud?
One of the significant features of molecular clouds is their concentration in the spiral arms of the Milky Way Galaxy plane. The uneven and bifurcating arms have no clear limit, yet when viewed from another galaxy, the nebulae in other spiral galaxies are spread out along these tiny lanes and form a symmetrical system. The nebulae are very close to the galactic plane; most are within 300 light-years, only 1% of the sun’s distance from the center. The high density of the stars within these arms supplies sufficient gravity to hold the gas to them.
What are ways a giant molecular cloud can be triggered to gravitational collapse?
A giant molecular cloud is supported against its own gravity by its internal pressure. This pressure has two sources:
- Gas pressure from internal heat
- Pressure from embedded magnetic fields
If the gravity grows larger than the internal pressure, the entire cloud will collapse. There are possible ways to trigger this including:
- Cloud-cloud collisions
- Passage through a spiral arm of the galaxy
- Shocks from nearby supernova explosions
How big is the Orion Molecular Cloud Complex?
A massive molecular cloud of interstellar gas and dust in the direction of Orion roughly centered on the Orion Nebula, is known as the Orion Molecular Cloud Complex. It is around 1,450 light-years away and measures 240 light-years across.
The Complex consists of five main components — Orion A and B, which are two giant molecular clouds, Orion OB1 Association, Lambda Orionis Molecular Ring, which is centered on Meissa, and the Orion-Eridanus Superbubble.
How do astronauts infer the presence of magnetic fields in a molecular cloud?
Magnetic fields in the universe are not visible to the naked eye and to most telescopes—plus, the interstellar magnetic field is about 10,000 times weaker than Earth’s magnetic field, that’s why measuring magnetic strength is extra difficult to gauge. But data from NASA’s Stratospheric Observatory for Infrared Astronomy (SOFIA)—a specially created Boeing 747 airplane that flies into the stratosphere and observes the universe using an onboard, high-powered camera called HAWC+ can capture infrared wavelengths of light. This can be used to see dust grains as they align perpendicular to the magnetic fields, allowing us to understand the strength and direction of the magnetic fields in molecular clouds.
