Mysteries of Astronomy: 15 Unanswered Questions

by Charles Fisher

May 9th, 2013 - Independent Scholars Evenings

Science has opened the skies to human curiosity, revealing grand designs and titanic forces beyond comprehension. Presented are 15 great mysteries of the cosmos, starting from our own solar system and moving into our galactic local group to the cosmological horizon beyond.

1. Why is there a giant hexagon on top of Saturn?

First noticed by the Voyager missions in 1988, the striking hexagon at the North pole of the planet Saturn has been captured in great detail by the Cassini orbiter, even to the extent of brief motion pictures. Each side of the hexagon is longer than the diameter of Earth. The longevity of the object is unknown.

Below is an original photograph from Voyager capturing the hexagon in the late 1980s.

Fluid dynamics researchers have long been able to create geometric shapes with tabletop demonstrations of fluid systems, providing a tentative explanation for the phenomena.

Perhaps equally surprising is the stationary hurricane at the center of the hexagon, similar to terrestrial hurricanes but on a much larger scale. Saturn's North pole has only just emerged from a long period of darkness making new images of this structure possible.

2. Why do some planets in the solar system lack magnetic fields (Mars, Venus)?

One enormous problem with life on Mars is that the planet lacks a magnetic field, allowing much more radiation to reach the surface.

[] The Earth is largely protected from the solar wind, a stream of energetic charged particles emanating from the Sun, by its magnetic field, which deflects most of the charged particles. These particles would strip away the ozone layer, which protects the Earth from harmful ultraviolet rays. Calculations of the loss of carbon dioxide from the atmosphere of Mars, resulting from scavenging of ions by the solar wind, are consistent with a near-total loss of its atmosphere since the magnetic field of Mars dissipated.

The conventional explanation for the lack of a field is that Mars is much smaller than the Earth, and the core of Mars is unable to sustain a "magnetic dynamo" which is thought to generate these fields, which requires a fast spin and retained internal heat according to the theory.

However, the planet Mercury is even smaller still than Mars, and it has a strong magnetic field for its size, so something in this explanation is not quite right.

3. How many more dwarf planets will be found that are similar in size to Pluto?

Not many people were happy with the recent decision to "demote" the planet Pluto to a "dwarf planet," but there were sound reasons for this action on the part of astronomers. Pluto's moon Charon was only seen for the first time in 1978, when it was realized that Pluto was much smaller than had originally been thought.

The discovery in 1801 of the dwarf planet Ceres begins the question of the classification of planets. Ceres is much smaller than Earth's moon, and has been the subject of much historical argument.

By 2005, Quaoar, Sedna, and Eris were objects known to be of similar size to the suddenly-less-massive Pluto, with Eris ultimately proving to be more massive.

At this point, the recognized dwarf planets are Ceres, Pluto, Haumea, Makemake, and Eris. Because these objects can be so difficult to see, there may be hundreds, or even thousands more of similar size.

4. Why are there no red dwarf stars of low metallicity?

Moving away from our solar system to our closest neighboring star, the dim red dwarf Proxima Centauri opens fundamental questions.

Red dwarf stars are very small, and are only able to convert hydrogen into helium very slowly. They are not able to continue a fusion chain with helium, because they are not massive enough. Helium is "ash" in a red dwarf, an inert material (larger stars are able to convert to a helium-burning phase, and stars of sufficient mass are able to move up the periodic table, ending at the last stage of exothermic fusion - the element iron).

The major theories behind the "Big Bang" assert that hydrogen, helium, and a small amount of lithium were all the elements that emerged from the birth of the universe. The process of the birth and death of stars builds upon these raw materials to form the higher elements:

An enormous question behind red dwarf stars is the lack of any examples that are pure, without contamination of higher elements seen by their emission spectra:

[] One mystery which has not been solved as of 2009 is the absence of red dwarfs with no metals. (In astronomy, a metal is any element heavier than hydrogen or helium.) The Big Bang model predicts the first generation of stars should have only hydrogen, helium, and trace amounts of lithium. If such stars included red dwarfs, they should still be observable today, but none have yet been identified. The preferred explanation is that without heavy elements only large and not yet observed population III stars can form, and these rapidly burn out, leaving heavy elements which then allow for the formation of red dwarfs. Alternative explanations, such as the idea that zero-metal red dwarfs are dim and could be few in number, are considered much less likely as they seem to conflict with stellar evolution models.

5. What causes Type 1A supernovas, accretion or collision?

There are two varieties of supernova explosions - those lacking hydrogen, and those exhibiting it. In Type 1 supernovas, the hydrogen emission spectrum is absent. In examples of Type 2, it is found.

A Type 1A supernova always involves the corpse of a star that has consumed all of its "fuel." A precursor for a Type 1A event is a high-mass white dwarf - a dead star with a heart of "ash." The usual ash for a Type 1A is a carbon-oxygen core.

The core has a pressure "tipping point" which is known as the Chandrasekhar Limit, a measure of the maximum limit of "electron degeneracy pressure." When a dead star with a carbon core reaches and exceeds 1.4 solar masses, a flash-fusion reaction ensues that blows the star entirely apart.

These explosions are always of similar intensity and luminosity, and they produce a similar characteristic emission spectra. Because they are so uniform, they are known as "standard candles" - they are used to measure distance based upon their redshift caused by dark energy, which will be discussed below.

But because they are used as standard candles, it is important to grasp the mechanisms of action that produce them, and this is not yet completely understood. [] "Despite widespread acceptance of the basic model, the exact details of initiation and of the heavy elements produced in the explosion are still unclear."

The accepted view of Type 1A explosions is that a single progenitor white dwarf slowly acquires mass from some source - usually, a companion star that is shedding mass at the end of its life in the "red giant" phase.

The accepted view has now been called into question. Evidence has been produced that indicates the collision of two white dwarfs is responsible for most of the Type 1A explosions that we see.

This issue may be of more than academic interest, as the nearby star Pyxidis seems nearly ready to become a Type 1A, and there has been some question of the potential danger it may present to Earth, due to its proximity.

6. How do stars larger than 8 solar masses form?

Large stars are beautiful and brief. The most famous "local celebrity" for a supermassive star is Eta Carinae:

Eta Carinae has reached the end of its life with a heart of iron. Since it has so much more mass than a white dwarf star, it will explode in a Type 2 supernova (since much non-burned/fused hydrogen remains on the surface). The iron core will be compressed, and the death spasm of this iron mass will force it into new states of matter - it may become a "neutron star" (perhaps for a very brief time), or, more likely, it will pass this final limit of matter and become a "black hole," a state of mass that is "undefined" in the current understanding of physics. A general view of Eta Carinae's composition as a large star nearing the end of its life is below:

The life of large stars is even more mysterious because, according to current theory, they should not exist. The outward-facing radiative pressure is larger than the force of gravity when the star reaches 10 solar masses, so enormous stars should not be possible. As it is, they are hardly rare. Why?

7. What were the "little green men" signals detected by the Arecibo Observatory?

The Arecibo Observatory is the largest (single-aperature)radio telescope in the world.

Completed in 1963, the telescope detected repeating, periodic radio pulses from a variety of sources that the researchers numbered with the prefix "LGM" - for Little Green Men.

...there was the unsettling possibility that beings -- which she and Hewish called Little Green Men -- were sending a transgalactic greeting.

"The press caught wind of it -- and somehow caught wind of the idea that it might be signals from little green men -- and they descended," Bell Burnell says. "And when they discovered that S.J. Bell [her first name is Susan] was a female, they descended even faster."

They asked Hewish about the astrophysical significance of the discovery. And then they turned to Bell Burnell for the "human interest" side: her body measurements, how many boyfriends she had, if she would undo a few shirt buttons for the photos.

"They just did not know what to make of a young female scientist," says Bell Burnell. "That's just the way the world was."

The objects that had been discovered were corpses, the strangest types known. Too massive to be white dwarfs, they are the last visible stage of solar death - a "neutron star."

An iron core constrained by the mass of a star pressing in on it can undergo the ultimate known compaction - electrons and protons within the core will combine, producing a highly compact "neutron superfluid" form of matter that crams the whole of a sun into the space of New York City.

Usually these objects emerge by exploding their surface layers away during their birth, creating beautiful structures, the most famous of which is the Crab Nebula:

But the type of neutron star found at Arecibo, and coincidentally the same type that produced and resides in the Crab, is very special. It is known as a "pulsar" - a neutron star that emits light from its magnetic poles (or perhaps radio waves, or x-rays, or other types of electromagnetic radiation). Furthermore, the Crab pulsar is elite among elite, being a very rare optical pulsar which can be seen in the visible spectrum.

The magnetic poles of a pulsar may not be aligned with the rotational poles, so the star appears to "blink" at regular intervals. Each "blink" is a complete rotation of the object, and there may be thousands of blinks per second.

The physics of neutron stars are not well-understood. It is unknown why only a small percentage of neutron stars become pulsars. The maximum mass of a neutron star, before it decays into a black hole, is only generally defined (by work done, in part, by Robert Oppenheimer). More exotic states of quark matter within a neutron star are described by theory, but the strange stars predicted have not been observed.

[] Though [the] very general picture of pulsars is mostly accepted, Werner Becker of the Max Planck Institute for Extraterrestrial Physics said in 2006, "The theory of how pulsars emit their radiation is still in its infancy, even after nearly forty years of work."

The mystery of the "little green men" may be solved, but the deeper mystery of what creates and drives these objects may never be known.

8. Why are intermediate-mass black holes so rare?

"Black holes are where God divided by zero."

- Albert Einstein

When enough mass is present to compress the iron core of a dying star, even the neutrons are overcome, and the collapse passes a point of no return. An "event horizon" is formed as the core is lost forever. The event horizon defines the boundary from which not even light can escape. The event horizon of a black hole is the point of no return.

Just as large stars are not rare, the black holes that form their corpses are also not rare. Finding a black hole of a few solar masses is not a great surprise. There are several in our immediate stellar vicinity.

Finding enormous black holes is also not difficult; such a monster seems to anchor most galaxies. Our own supermassive black hole that anchors the Milky Way galaxy (in which we reside) has been named "Sagittarius A-Star" and is abbreviated Sgr A*. It is a relative light-weight at 4 million solar masses.

What is difficult to find are black holes that are of intermediate mass. Finding black holes in general is not easy - they must be inferred by the movements of other objects, since they cannot be seen. But black holes in the range of thousands of solar masses are very, very rare.

But this is not to say that they are entirely absent:

[] In November 2004 a team of astronomers reported the discovery of a potential intermediate-mass black hole, referred to as GCIRS 13E, orbiting three light-years from Sagittarius A*. This black hole of 1,300 solar masses is within a cluster of seven stars. This observation may add support to the idea that supermassive black holes grow by absorbing nearby smaller black holes and stars.

9. Why do most galaxies have a supermassive black hole at their core?

Regular galaxies, of the spiral or elliptical type, are usually defined by the black hole that anchors them. They can be lost by collisions or impacts with other galaxies, but they form a regularly observed feature of the structure.

[] Every bright elliptical galaxy is believed to contain a supermassive black hole at its center. The mass of the black hole is tightly correlated with the mass of the galaxy, via the M-sigma relation. It is believed that black holes may play an important role in limiting the growth of elliptical galaxies in the early universe by inhibiting star formation.

[] [In spiral galaxies]... Many bulges are thought to host a supermassive black hole at their centers. Such black holes have never been directly observed, but many indirect proofs exist. In our own galaxy, for instance, the object called Sagittarius A* is believed to be a supermassive black hole. There is a tight correlation between the mass of the black hole and the velocity dispersion of the stars in the bulge, the M-sigma relation.

No one knows why.

10. What causes relativistic jets?

When a black hole feeds, two structures form around it. An "accretion disc" of swirling material be drawn from the inflowing mass into a planar vortex. The disc material will heat as it is compressed, accelerated, and drawn to the hole. The edge of the disc, immediately before the event horizon, will be hot enough to emit X-rays.

The other structure defies explanation. Two polar jets will form, allowing a small amount of material to escape at high velocity and energy.

Should the influx of material cease, the structures will immediately collapse.

No one knows why.

11. Why are quasar-class active galactic nuclei only found in the distant, older universe?

Polar jets in their forms can be the brightest objects in the sky. Galaxies are essentially stationary to us since it takes so long for them to move significantly. They are arranged haphazardly around us. Some of these galaxies have central black holes that are feeding, and when they do, we see enormous jets. Let's examine their forms:

We would call the first example above a "radio galaxy," the middle examples would be "quasars," and the last example would be a "blazar" (where we look right down the jet).

We don't find these galaxies with brilliant polar jets close to us; we only see them very far away. Since light travels at a relatively slow speed compared to the distances involved, we are seeing them far back in the past. So why don't we see any close to us? Why aren't there any nearby galaxies with an enormous black hole that is quenching its appetite?

No one knows.

12. What causes the intricate structure of a spiral galaxy?

The graceful structures of well-formed spiral galaxies are aesthetically enchanting, a feast for the eyes:

Beautiful as these objects are, there is no real explanation for why they have formed. There are some preliminary computer simulations available, but they are far from conclusive.

13. What is dark matter, and how does it hold galaxies together?

Despite their beauty, when astronomers add up all the mass in a galaxy, they are led to a dramatic conclusion: it should fall apart. There is far too little gravitation, and the mass behind it, to hold a galaxy together.

The science community is desperate for an explanation. Grand searches have begun for "Weakly-Interacting Massive Particles" (WIMPs), but the honest answer to the question of how any and all galaxies hold together is short and sweet:

No one knows.

14. What is dark energy, and how does it push everything apart?

Imagine that you and your friends are all ants, and that you live on a rubber band.

Now imagine that someone pulls the ends of the rubber band hard.

If there are a few of you huddled together on a small section of the rubber band, you can hold on to one another and keep one another's company. However, you may see your friends some distance away from you begin running towards you, but no matter how hard they run, they still get farther away, such is the force with which they are being pulled away from you.

Such is what is happening with all galaxies, U.S. astronomer Edwin Hubble discovered. Hubble's Law and the associated "Hubble constant" are the conclusive science behind the observed expansion of the visible universe.

The further we are away from some distant object, the more rapidly it is moving away from us. Knowing the expansion factor, and making use of the "Doppler shift," any observation of the previously-mentioned "standard candles" should give us characteristic light curves shifted in frequency by an amount comparable to the distance of the object from us.

The ants at the far ends of the rubber band are moving away from one another more quickly than ants that are closer to the middle. In a similar way, some points far distant from us are moving away faster than the speed of light. These points are said to have departed our "cosmological horizon," and the universe that we can actually see becomes less and less of the universe that actually exists.

And if this were not shocking enough, Hubble's constant is increasing:

[] A value for "q" measured from standard candle observations of Type Ia supernovae, which was determined in 1998 to be negative, surprised many astronomers with the implication that the expansion of the universe is currently "accelerating" (although the Hubble factor is still decreasing with time, as mentioned above in the Interpretation section; see the articles on dark energy and the .CDM model).

So why is the universe being stretched out underneath us, pulling everything we see inexorably out of our reach?

No one knows.

15. Is there life elsewhere? Where is everybody? What is the Fermi Paradox?

With billions of galaxies in the sky above us, and billions of stars in every galaxy, why can we see no evidence of intelligent life? Regardless of the rarity of life, there are so many stars before us that many think of playing slots in Vegas with every dollar that was ever printed: you're bound to win something.

Such was the question asked by Enrico Fermi with refreshing simplicity: "Where is everybody?!"

Why are we alone? Why can we see nothing? Why is no advanced civilization visible despite an insurmountable gulf of distance across which we could never communicate? The odds of our solitude are ridiculously small, and yet we are alone.

And no one knows why.