A Star called mystery : what we still don’t know about the Sun

(ORDO NEWS) — The sun is incomparably closer to us than any other star. It is only eight light-minutes away, while Proxima Centauri is over four light-years away. It would seem that we should know everything about the Sun and even more. However, it was not there.

Famed astronomer Fred Hoyle once remarked, “In principle, a star has a fairly simple structure.” His colleague Professor Redman retorted, “You’d look pretty simple too, Fred, from ten parsecs away.”

Indeed, studying the Sun from a distance of millionths of a parsec convinces astronomers that our star is not at all simple (and if so, then others are hardly simpler).

The sun is a dazzling challenge in every way to our ability to experience the world. It is being closely studied in every possible way, from good old optical observations to solar neutrino trapping, and yet it still holds many mysteries.

Ordinary but familiar

I would not like to create the impression that the Sun is a solid white spot in the scientific picture of the world. Of course, we know a lot about him. Even in a very compressed form, this knowledge, if written down in one book, would have amounted to several weighty volumes.

Let’s try to list the most important thesis. The Sun is more than a hundred times larger than the Earth in diameter and 330,000 times larger in mass. The temperature on its surface is 5500 degrees Celsius, and in the center – 15 million.

The sun is about five billion years old. The star’s energy source is thermonuclear reactions that convert 600 million tons of hydrogen into helium every second.

This fuel will be enough for the star for about another five billion years, that is, now it is in the middle of its life path. In general, the Sun is a typical star of the spectral class G, average in all respects.

The work of the Sun as a “thermonuclear heater” is quite well studied – the main process to which the Sun (and at the same time we) owe their existence. Thermonuclear reactions take place in the center of the star.

These reactions produce photons. The pressure of this radiation literally bursts the Sun from the inside and does not allow gravity to compress it.

The path of a photon to the surface (photosphere) takes tens of thousands of years: it is absorbed and re-emitted by matter countless times. Having reached the photosphere, the light finally breaks out into the expanses of space, including in order to warm the Earth.

But this, so to speak, is the main activity of the luminary. It comes with a lot of side effects. And they are often mysterious.

A Star called mystery what we still dont know about the Sun 1
The bright dots detected by the Solar Orbiter probe could be corona-heating nanoflares. May not be

Crown question

One of the most famous mysteries of the Sun has to do with the temperature of the corona. The corona is the outer rarefied layer of the Sun’s atmosphere. It is the farthest from the central heat source and, it would seem, should be relatively cool.

But it was not there. The temperature of the layer underlying the corona – the chromosphere – is measured in tens of thousands of degrees. But in a thin (some hundreds of kilometers) transition layer between the chromosphere and the corona, the temperature suddenly rises to millions of degrees!

Why? What warms the crown? An endless crackling firework of microscopic flashes? Or an electric current flowing through the plasma? Or maybe waves like sound (for sophisticated readers, let’s clarify: magnetohydrodynamic)?

All three hypotheses have very solid supporters in the scientific world. This means that no one knows for sure. Periodically, the media circles the news with a headline like “Scientists have finally revealed the secret of the high temperature of the corona.”

They would be more exciting if this secret had not been “revealed” with enviable regularity for many decades.

The wind knows

Another huge mystery in every sense is the solar wind. It is a stream of charged particles, mostly protons and electrons, constantly flowing out of the Sun.

There are two types of solar wind: slow (300-400 kilometers per second) and fast (700-800 kilometers per second). Isn’t it true that even the “slow” wind is not so leisurely?

The solar wind literally fills the solar system. True, it is very rarefied: there are only 5-10 particles of the solar wind per cubic centimeter of near-Earth space.

For comparison, there are more atoms in a glass of water than there are glasses of water in the oceans. What for an astronomer is a flow of matter, an earthly physicist will rightfully call a vacuum.

The main thing we don’t know about the solar wind is whether we should be surprised at its existence. On the one hand, the corona is very hot, although it is not known why. It seems natural that some of the matter escapes from it into space, like steam from a saucepan.

But on the other side of the scale is the colossal gravity of the Sun. And we do not know who should win this confrontation. There are simply no good equations describing the flow of the solar wind. And those that are, are solved in both directions.

In other words, they describe both the outflow of matter from the star and its fall onto the star. If astronomers ever discover Sun 2.0, it’s entirely possible that it will absorb surrounding matter rather than spray its own into space.

However, the problem can be solved by adding an additional source of energy to the solar wind. That is, assuming that the substance leaves the corona not only due to “evaporation”: something additionally pushes it out. If only we knew what exactly… What about plasma waves in the corona?

Another mystery of the solar wind is its turbulence. It does not flow in calm streams like a flat river. Instead, the plasma boils and almost foams like a mountain stream. Why? Probably, this is somehow connected with the mechanism of its expiration – it is a pity that we do not know it yet.

Restless luminary

Astronomers also have questions about the magnetic field of the Sun. True, there is nothing surprising in the fact that the star has this field.

The magnetic field is generated by electric currents, that is, by the ordered movement of charges. And the luminary for the most part consists not of neutral atoms, but of “naked” protons and electrons. It is worth bringing them into an orderly movement, and you get a current.

Surprisingly, the Sun’s magnetic poles drift so fast that every 11 years the north and south poles change places. In other words, there is a 22-year cycle: the north magnetic pole returns to the same place every 22 years.

Why does this happen at all and why is the period exactly 22 years? Of course, there are models that answer this question, how could it be without them. But they still have a lot of unexplained details.

The famous 11-year cycle of solar activity is associated with the 22-year cycle of changing poles. Its best indicator is spots (small areas of low temperature by the standards of the Sun).

As long as the Sun’s magnetic poles are close to the rotation poles, the number of sunspots on our star is minimal – this is the minimum of an 11-year cycle.

As the magnetic poles drift towards the equator, the number of sunspots increases. It reaches its maximum when the magnetic poles of the star are at its equator – this is the maximum of the 11-year cycle.

Having passed the equator, the magnetic poles again approach the geographic ones, only now to the opposite ones. And the number of spots again falls to a minimum.

Thus, an 11-year activity cycle is half a 22-year polarity reversal cycle. Not only spots, but also flashes, emissions of matter into space and other phenomena obey him.

It is all the more annoying that we do not really know the nature of this cycle. Adding to the mystery is that the cycle is not strictly regular: lows and highs are sometimes unusually deep or long.

A Star called mystery what we still dont know about the Sun 2
Coronal mass ejections are one of the most impressive manifestations of solar activity

When the sun spits

Solar activity in general is full of secrets. Of all its many manifestations, let us choose the phenomenon that has, perhaps, the most noticeable impact on our planet. These are coronal mass ejections or CMEs. CMEs are billions of tons of plasma ejected from the Sun.

They are denser than the background solar wind, but still very rarefied by Earth standards. In terms of volume, such clouds significantly exceed the Earth and regularly cover it “with its head”. Such “bathing” can cause a magnetic storm.

Therefore, CMEs are of interest not only to astronomers who are far from earthly concerns, but also to practitioners.

Statistics show that coronal mass ejections are closely associated with flares. Strong flares are often accompanied by CMEs and, consequently, by magnetic storms (which draws the attention of the public to them, which is not always proportionate).

But what is the nature of this connection? Again, a mystery. Maybe the flash is causing the blowout? Or vice versa, is it called? Or maybe the flash and the ejection are two consequences of the same cause? As usual, all three hypotheses have supporters.

It would seem that it is enough to look at what happens first: a flash or a release of matter. But even on this point, the observational data are surprisingly contradictory. In general, it seems that it happens this way and that.

In general, for whatever you take – whether for the crown, for the solar wind or solar activity – riddles will fall like a bag. It seems that the Sun will provide work for more than one generation of researchers.


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