The Interstellar Medium
At 153 AU (153 times the average distance from Earth to the Sun) and counting , Voyager 1 is a long, long, long way away. And at these astronomical distances there are things that can be observed which can’t be observed with precision within the reach of the Sun (the heliosphere), namely the interstellar medium.
This is made up of all the low density gas and dust that fills the area between stars: 99% interstellar gas and 1% interstellar dust, making up 10-15% of the Milky Way visible from Earth . And this concoction of material has recently been directly observed by the Voyager 1 probe, but before we delve into the findings, let’s have a look at the science behind the interstellar medium.
Survey of the ionised gas in the Milky Way, surveyed by WHAM (Wisconsin H-Alpha Mapper)
Credit: Haffner, L, et al. (2003), funded by the NSF
The lion’s share of the interstellar medium is taken up by interstellar gas: a mixture of some helium (10-25%) and a lot of hydrogen (75-90%). However, it’s not the even and perfect spread of the odd atom or two in a partial vacuum that you might expect.
Instead, both temperature and pressure can vary vastly, with temperatures ranging from just a few ºC above absolute zero (the coldest temperature, where no energy is released), to millions of ºC above absolute zero (i.e. millions of Kelvin, shortened to K); and pressures ranging from just one atom in a cubic centimetre of space, to enough atoms to block the light behind it.
There are generally four forms of interstellar gas, the first being cold (around 100K), neutral hydrogen clouds (called HI regions, pronounced “H-one”). This is what most interstellar gas is, but due to its cold temperature, it’s not the most visible, since there needs to be some energy to emit light. However, there’s help in the form of absorption, where particular wavelengths of light passing through the cloud are absorbed, leaving characteristic absorption lines (gaps) in the spectra of light emitted from stars (creating absorption nebulae when in high concentrations).
The Lockman Hole, where redder light is taken out by the HI region to give astronomers a clearer view of the bluer light.
Credit: Chandra X-Ray Observatory, NASA
Then theres the hotter (10,000K) ionised hydrogen gas clouds (called HII regions, pronounced “H-two”). Like HI regions, HII regions absorb light, but because HII regions can absorb so much energy from near by hot stars, they emit light (through a process of electrons loosing the energy they absorbed from nearby stars and emitting photons -light), creating characteristic emissions lines (most notably the red H-alpha, Hα, line) in the spectra of these objects (making emission nebulae when in high enough concentrations).
The Orion Nebula (an HII region and emission nebula).
Credit: NASA/ESA/M. Robberto (Space Telescope Science Institute/ESA)/Hubble Space Telescope Treasury Project Team
Then there’s the even hotter interstellar gas (millions of Kelvin) made of not just hydrogen but some oxygen, and not even the normal oxygen we breathe: oxygen with 5 electrons missing. What’s more, the gas emits not low energy red Hα light, but very high energy X-rays. But how? The answer lies in the high energy supernovae explosions (a stage towards the end of a star’s life) which eject gas at fast speeds of tens of thousands to millions of kilometres per hour, heating the interstellar gas it comes into contact with and creating these high energy gas clouds.
Vela Supernova Remnant (one of the largest x-ray features in the sky)
Credit: Harel Boren, CC BY-SA 4.0
Extremely cool (10K) molecular gas clouds are the last type, made up of not individual atoms, but molecules such as CN (cyanide, made of carbon and nitrogen) and CH (hydridocarbon, made of carbon and hydrogen), as well as CO (carbon monoxide, made of carbon and oxygen) and many other molecules. Like the HI regions, they absorb light at specific wavelengths which correspond with the atoms present, and, if the gravitational pull is strong enough, these regions can grow into complex and colourful clouds of interstellar gas .
Taurus molecular cloud, glowing due to star formation within the cloud
Credit: ESA/Herschel/NASA/JPL-Caltech/R. Hurt (JPL-Caltech), CC BY-SA 3.0 IGO
However, how can these interstellar gas clouds contain molecules when they’re bombarded by high energy rays such as ultra-violet (UV) light which splits apart such molecules? The answer lies in the important and complex world of interstellar dust; the next stop on our journey.
Unlike the dust you tried to get rid of when you were doing a bit of spring cleaning, this dust is a bit more ‘space-age’. Interstellar dust (not to be confused with interplanetary dust or galactic dust) is made of carbon, iron compounds, silicates, and/or ice, measuring just 0.1 microns (a 750th of the width of a human hair) across and taking irregular shapes.
Due to the solid form of dust, if it collects into high enough densities, it can scatter the light through extinction. This is where the size and density of the dust comes into play, with most interstellar dust being the size which can scatter bluer light (such as the UV light which can split apart molecules), removing this light to create an appearance that the dust is red: interstellar reddening (not to be confused with redshift). This then protects those all important gas molecules in molecular gas clouds.
Barnard 68, also known as the Bok globule, shows interstellar reddening through the red infrared section in the centre
Credit: ESO, CC BY 4.0
However, if it is in high enough concentrations (100-1000 grains per cubic centimetre), then all the light from behind the dust is absorbed, creating dark nebulae (seen as patches of perfectly dark sky with no stars).
Barnard 56, also known as the Pipe Nebula, is a dark nebula
Credit: ESO, CC BY 4.0
The dust doesn’t just affect the light passing through it, but also the light being directed onto it. It reflects this light, creating reflection nebulae .
Reflection nebula Messier 78 in Orion
Credit: ESO/Igor Chekalin CC BY 4.0
However, we don’t just observe these wonderful and complex nebulae from afar; there’s a cloud going through our own Solar System right now. Aptly known as the Local Galactic Cloud, it creates a constant flow of particles, known as interstellar wind, coming from the middle of the constellations of Sagittarius, Scorpius, Libra and Ophiuchus. This then shows the direction of the movement of the Solar System relative to the cloud, which we’re moving through at 80,000kph (with the location of the source of wind having changed by 4 to 9 degrees within 40 years, possibly due to the fact we’re near the edge of the cloud and may be experiencing ‘turbulence’) .
Direction of the interstellar wind. Credit: NASA/Goddard Space Flight Centre
But wait… nebulae? How come interstellar dust and gas can get into these intricate formations instead of becoming an even spread throughout the Universe?
The final piece to this interstellar puzzle lies in stellar wind (not to be confused with interstellar wind): a force which interacts with and shapes in the interstellar medium.
It is made up of the high-energy protons and electrons ejected by stars across the Universe, including our own Sun. This fast-moving and energetic stream of particles then ‘blows’ the interstellar dust and gas about to then create the structures we observe today. However, along with interstellar wind, supernovae also play a key role, moving high energy particles at high speeds through the interstellar medium, shaping it and creating HII regions.
However, as well as shaping these nebulae, stellar wind adds dust and metals to the interstellar medium, and, along with the numerous stars that ‘die’ and shed their material, help the formation of new stars through creating an environment where gas and dust can compress to trigger star formation .
The Pillars of Creation (the purple points throughout are new stars forming)
Credit: ESO, CC BY 4.0
Observations from the Edge
This is where we pick up the story of Voyager 1 and the recent news stemming from its observations. It used its onboard plasma wave instrument to detect the minute densities of plasma (ionised gas, in this case being charged interstellar gas) it’s passing through (unlike previous observations which only looked at specific high energy events).
The frequency of the audible signals produced by the plasma suggest an increase in plasma density. This is an observation of specific waves of ‘turbulence’ in the interstellar medium, produced by our galaxy’s rotation, as well as supernovae and solar eruptions (such as Coronal Mass Ejections or CMEs). These measurements will then help scientists map out the interstellar medium around our own Solar System .
The continuous weak signals (in red) detected by Voyager 1's Plasma Wave Subsystem and filtered through by Stella Ocker
Credit: NASA's Voyager 1 Plasma Wave Subsystem/Stella Ocker
Hopefully, with much more data from Voyager 1 and 2 to be sent back and processed by scientists on Earth, we will begin to build an accurate picture of how the interstellar medium behaves and what processes drive it.
by George Abraham, ADAS member.
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Click here to find where Voyager 1 and 2 are currently
Click here to see ESO's 3D visualisation of the Pillars of Creation
Click here to view the paper published in Nature Astronomy about the detections of waves in the interstellar medium
Click here to look at the sky in Hα
Click here to look at the Vela Supernova remnant in X-rays, here to look at the Lockman Hole in X-rays, here to look at the Orion Nebula in Hα, here to look at Barnard 68 in near-infrared, here to look at the dark Pipe Nebula, here to look at the reflection of M78 and here to look at the star formation in the Pillars of Creation, all on ESA Sky
Click here to listen to the hum of the Universe taken back in 2013
"The Vela Supernova Remnant and Surrounds - a Photographic Tour". Pretoria Centre. Archived from the original on 15th May 2021.