The Space Between Us

From our vantage point on Earth, we tend to think of our surroundings as the norm of the universe.  It is not.  When we study astronomy we focus on the planets, the Sun, stars and galaxies.  These objects represent a small fraction of the universe.  If you could shrink the Sun to the size of a grain of sand, the nearest star would be another grain of sand over four miles away.  On this scale, light would travel at seven inches an hour.  What lies in all that space between the stars?  A cauldron of plasma, dust, gas, and magnetic fields in conditions we do not experience on Earth.  Some of the most important processes in the universe occurs in these environments.

Plasma is electrified gas.  In the Sun, or in any star, the heat of the core separates positively charged nuclei (ions) from negatively charged electrons.  These free floating particles are then discharged into space via the solar wind.  Plasma does not occur naturally on the Earth’s surface although it can be created to be used in florescent lights and plasma TV’s.  As plasma carries an electrical charge, its movement is determined by the ambient magnetic field.  A charged particle travels along the path of a magnetic field line.  In turn, the solar wind drags the solar magnetic field towards the planets.  This is referred to as the Interplanetary Magnetic Field or IMF.  As a result of the Sun’s rotation, the IMF is spiral shaped much like water from a rotating sprinkler.  The IMF also undulates in a wave-type formation as the image below indicates.

Parker Spiral, Credit: NASA/J. Jokipii, University of Arizona.

The journey of this plasma is fairly uneventful until it collides with a planet.  In the case of Earth, the IMF connects with the Earth’s magnetic field to transfer mass (the plasma) and its energy.  Once this plasma enters the Earth’s magnetic field, it follows the Earth’s magnetic field lines eventually finding its way into the upper atmosphere near the magnetic poles.  Here, these highly energetic particles collide with oxygen and nitrogen atoms.  The kinetic energy of these collisions excites the atom’s electrons to a higher energy level.  The electrons eventually fall back to their original energy levels and release the energy in the form of light causing the aurora.  Without this protective shield, life could not exist on Earth’s surface.  And that scenario is played out on Mars which lacks a magnetic field.

Most of the solar wind does not collide with planets.  What becomes of it?  Eventually, it hits the heliopause.  Here is where the solar wind meets the interstellar medium and no longer has the ability to push out any further.  Both Voyager I & II, launched in 1977 and still sending data back to Earth, are headed towards the heliopause.  Both have crossed the termination shock which precedes the heliopause.  It is here where the solar wind slows from supersonic to subsonic speeds.  It is not known specifically when the Voyager’s will cross this threshold into the interstellar medium.  But hopefully, it will occur before the Voyager’s last instruments are shut down in 2025.  What do we know about the interstellar medium?

Voyager’s golden record. The hydrogen electron spin state is depicted on the lower right. This provides a calibration for distance and time for any extraterrestrial life that might encounter Voyager. A detailed explanation of the golden record can be found here.  Credit: NASA/JPL

As hydrogen was created in the immediate aftermath of the Big Bang, it is the most common element in the universe.  In interstellar space, it is not hot enough to ionize hydrogen.  Neutral hydrogen (HI) emits 21 cm radio waves.    If the spin of a hydrogen atom’s electron and proton are parallel, the electron flips its spin to be anti-parallel.  This action causes the electron to occupy a slightly lower energy level emitting radio waves in the process.  Unlike light, radio transmissions penetrate through dust clouds.  Think of it this way, if you have your radio on, the reception will be the same regardless how dusty your room is.  This has allowed astronomers to complete comprehensive maps of galactic hydrogen gas clouds.  This is crucial in mapping the Milky Way as hydrogen’s radio transmissions give us a better look at our home galaxy.

In a spiral galaxy, hydrogen tends to be found in the arms.  It is in these areas where stars tend to be born.  The rotational velocity of the hydrogen in the arms and its resultant red/blue shifts allow us to differentiate it from intergalactic hydrogen.  The 21 cm radio emissions are so ubiquitous that it was decided to use these emissions on the Voyager golden record as a calibration scale for potential extraterrestrials who might find the probe.  The thinking being, as this is the most common emission in the universe, any alien race would also know about it and use it to decipher the time and distance measurements.

A 360 degree map of hydrogen in the Milky Way. The oval represents a sphere flattened by making a cut from one pole to the other. Bluish gas is approaching Earth while greenish gas is receding from Earth. The plane of the Milky Way runs across the center while the neighboring Magellanic Clouds are in the lower right. Credit: HI4PI: A full-sky HI survey based on EBHIS and GASS, Astronomy & Astrophysics.

When there is a star near a hydrogen cloud, it can heat it up to the point where it ionizes.  When accelerated, charged particles emit radio waves.  In an ionized hydrogen cloud, when a negatively charged electron is near a positively charged proton, it accelerates and emits radio waves.  This is something akin to radio transmission towers on Earth.  Electrons are accelerated up and down the tower generating the radio transmission you receive at home.  Ionized hydrogen clouds are hot enough to emit visible light as well.  By combining radio and visual observations, astronomers have been able to map out the spiral arms of the Milky Way.

Orion Nebula as captured by the Hubble Space Telescope. During winter and spring it is visible with binoculars.

Hydrogen is not the only element in interstellar space.  The second most abundant element is helium, also created in the throes of the Big Bang.  Beyond that there are trace amounts of other elements such as oxygen and carbon generated in the nuclear fusion of ancient generation stars that released these elements as a planetary nebula or supernova explosion.  While small in amounts, these are large in importance.  This is especially true of organic, carbon based molecules in space.  It is these molecules that form the basis of life on Earth, and perhaps elsewhere.

Also occupying interstellar space are dust grains.  If lightwaves are smaller than the dust grains, it is scattered in random directions.  If a lightwave is longer than a dust grain, it is not scattered and allowed to pass through unabated.  Thus, on Earth, short wavelength blue light is scattered by dust in the atmosphere resulting in the blue sky.  Conversely, long red wavelength passes through dust and creates the red sky at sunset.  The same processes are at work in space.  Dust grains scatter blue light reddening celestial objects when viewed here on Earth.  In some cases, such as nebulae and the galactic center, dust can obscure our view entirely.  The answer is to view in even longer wavelengths than red light – infrared light.

Infrared light, which is basically heat, is not visible to the eye.  You cannot see body heat with your eyes, but you can view it with night vision goggles, which is an infrared detector.  At near-infrared wavelengths, located adjacent to the optical band on the electromagnetic spectrum, dust is transparent.  Far-infrared, which has longer wavelengths closer to radio waves, can detect dust formations that radiate in these wavelengths.  The Earth radiates most in infrared and thus, it is advantageous to have an infrared observatory in space protected from interference from Earth.  The Spitzer Space Telescope does just that by observing in the infrared.

A composite image in both far and near infrared showing stars in the Milky Way core and the dust that normally obscures those stars. Credit: NASA/JPL-Caltech

Dust grains are important for life.  When dust grains begin to clump together around a protostar, it is the first step in planet building.  It has been theorized that dust is how organic material was delivered to Earth to form the building blocks of life.  The early Earth would have been too hot for organics to survive on the surface.  The theory is ultraviolet radiation broke apart dust grains, allowing them to recombine into organic compounds to be deposited on Earth via asteroids and comets.  The jury is still out on this, and while we cannot observe the formation of the Earth, we can observe the formation of other planetary systems via infrared and radio observatories.  The James Webb Space Telescope, to be launched in 2018, will observe in the infrared and should advance our understanding of these processes greatly.

The interstellar medium has about one atom per cubic centimeter.  The intergalactic medium has less than one atom per cubic meter.  It is also very hot at 100,000 to 10,000,000 Kelvin.  This is not intuitive given the lack of obvious heat sources between galaxies.  We know the temperature as the intergalactic medium emits high energy x-rays indicative of hot objects.  The heat is generated by active galactic nuclei and gravitational wells of galactic clusters.  Temperature is a measure of energy which in turn is a measure of motion.  Since this space is so rarefied, it does not take a lot of push to move it to high velocities.  And since it is so rarefied, this is where dark energy makes its full impact.

The expansion of the universe slowed until about 5 billion years ago when dark energy became more dominant than gravity. Credit: Zosia Rostomian, Lawrence Berkeley National Laboratory, and Nic Ross, BOSS Lyman-alpha team, Berkeley Lab

Dark energy is the force that is speeding up the expansion of the universe.  Until about 5 billion years ago, gravity dominated the universe and was slowing down the acceleration.  Since then, dark energy has dominated and has accelerated the expansion.  What is dark energy?  We don’t know.  Within the confines of galaxies, gravity still dominates and we don’t feel the universal expansion on Earth.  However, these confines are but a small part of the volume of the universe.  The ultimate fate of the universe will be dictated by dark energy in the intergalactic void.  Some feel that the universe will end in a Big Rip where even subatomic particles are shredded apart by the continuing expansion of the universe.  Obviously, life could not exist in this state.  Worry not, this would be many, many billion years in the future.  Nonetheless, our universe has a life cycle.  And this underscores our need to understand the processes at work in seemingly, but not quite, empty space.

*Image atop post is Hubble wide field view of NGC 6791, an open star cluster that also has a couple of galaxies in the background.  Credit:  NASA, ESA, and L. Bedin (STScI)