Secrets of the Sun
Massive Solar Eruptions Could Threaten our High-tech Society

Secrets of the Sun
Massive Solar Eruptions Could Threaten our High-tech Society

In ‘Secrets of the Sun’, we explore the myriad mysteries which lie beneath the fiery surface of the sun. Our sun is a fireball in the sky - a bubbling, boiling, kinetic sphere of white hot plasma, exploding and erupting. Its size is almost unimaginable: one million Earths would fit within its boundaries.

In this violence is born almost all the energy that makes existence on Earth possible. Yet its full mysteries are only now beginning to be understood. From sun spots to solar eclipses, solar flares to solar storms, the birth of the sun to its potential death, discover the science and history behind this celestial object that makes life on Earth possible.

The Sun is the star at the center of the Solar System. It has a diameter of about 1,392,000 kilometers (865,000 miles), about 109 times that of Earth, and its mass (about 2 × 1030 kilograms, 330,000 times that of Earth) accounts for about 99.86% of the total mass of the Solar System.

About three quarters of the Sun's mass consists of hydrogen, while the rest is mostly helium.

Less than 2% consists of heavier elements, including oxygen, carbon, neon, iron, and others.

The Sun's color is white, although from the surface of the Earth it may appear yellow because of atmospheric scattering of blue light.

Its stellar classification, based on spectral class, is G2V, and is informally designated a yellow star, because its visible radiation is most intense in the yellow-green portion of the spectrum.

In this spectral class label, G2 indicates its surface temperature of approximately 5,778 K (5,505 °C; 9,941 °F), and V (Roman five) indicates that the Sun, like most stars, is a main sequence star, and thus generates its energy by nuclear fusion of hydrogen nuclei into helium. In its core, the Sun fuses 430–600 million tons of hydrogen each second.

Once regarded by astronomers as a small and relatively insignificant star, the Sun is now presumed to be brighter than about 85% of the stars in the Milky Way galaxy, most of which are red dwarfs. The absolute magnitude of the Sun is +4.83; however, as the star closest to Earth, the Sun is the brightest object in the sky with an apparent magnitude of −26.74.

Secrets of a Dynamic Sun

Five major discoveries about our Sun.
From NASA's Goddard Space Flight Center.

The Sun's hot corona continuously expands in space creating the solar wind, a stream of charged particles that extends to the heliopause at roughly 100 astronomical units.

The bubble in the interstellar medium formed by the solar wind, the heliosphere, is the largest continuous structure in the Solar System.

The Sun is currently traveling through the Local Interstellar Cloud in the Local Bubble zone, within the inner rim of the Orion Arm of the Milky Way galaxy.

Of the 50 nearest stellar systems within 17 light-years from Earth (the closest being a red dwarf named Proxima Centauri at approximately 4.2 light years away), the Sun ranks 4th in mass.

When observing the Sun with appropriate filtration, the most immediately visible features are usually its sunspots, which are well-defined surface areas that appear darker than their surroundings because of lower temperatures.

Sunspots are regions of intense magnetic activity where convection is inhibited by strong magnetic fields, reducing energy transport from the hot interior to the surface. The magnetic field causes strong heating in the corona, forming active regions that are the source of intense solar flares and coronal mass ejections.

The largest sunspots can be tens of thousands of kilometers across. The number of sunspots visible on the Sun is not constant, but varies over an 11-year cycle known as the solar cycle. At a typical solar minimum, few sunspots are visible, and occasionally none at all can be seen.

Those that do appear are at high solar latitudes. As the sunspot cycle progresses, the number of sunspots increases and they move closer to the equator of the Sun, a phenomenon described by Spörer's law. Sunspots usually exist as pairs with opposite magnetic polarity.

The magnetic polarity of the leading sunspot alternates every solar cycle, so that it will be a north magnetic pole in one solar cycle and a south magnetic pole in the next.

The Sun orbits the center of the Milky Way at a distance of approximately 24,000–26,000 light years from the galactic center, completing one clockwise orbit, as viewed from the galactic north pole, in about 225–250 million years.

Since our galaxy is moving with respect to the cosmic microwave background radiation (CMB) in the direction of constellation Hydra with a speed of 550 km/s, the sun's resultant velocity with respect to the CMB is about 370 km/s in the direction of Crater or Leo.

The mean distance of the Sun from the Earth is approximately 149.6 million kilometers (1 AU), though this varies as the Earth moves from perihelion in January to aphelion in July. At this average distance, light travels from the Sun to Earth in about 8 minutes and 19 seconds.

The energy of this sunlight supports almost all life on Earth by photosynthesis, and drives Earth's climate and weather. The enormous effect of the Sun on the Earth has been recognized since prehistoric times, and the Sun has been regarded by some cultures as a deity.

An accurate scientific understanding of the Sun developed slowly, and as recently as the 19th century prominent scientists had little knowledge of the Sun's physical composition and source of energy. This understanding is still developing; there are a number of present-day anomalies in the Sun's behavior that remain unexplained.

Faint Young Sun Problem

Theoretical models of the Sun's development suggest that 3.8 to 2.5 billion years ago, during the Archean period, the Sun was only about 75% as bright as it is today.

Such a weak star would not have been able to sustain liquid water on the Earth's surface, and thus life should not have been able to develop.

However, the geological record demonstrates that the Earth has remained at a fairly constant temperature throughout its history, and that the young Earth was somewhat warmer than it is today.

The consensus among scientists is that the young Earth's atmosphere contained much larger quantities of greenhouse gases (such as carbon dioxide, methane and/or ammonia) than are present today, which trapped enough heat to compensate for the smaller amount of solar energy reaching the planet.

Attack of the Sun

Massive solar eruptions could threaten our high-tech society.

The year was 2003 with Halloween approaching. 93 million miles away an angry sun began to vent its rage.

Dark regions, called sunspots, appeared unexpectedly on its surface, a sign of rising tension within.

It had been three and a half years since the sun last erupted in fury... the peak of an 11-year cycle of solar flare-ups.

Back then, we got ready for it by shutting down satellites that were vulnerable to high levels of radiation.

But no one expected this.

In what should have been a low point in solar activity, the sun erupted in a series of massive explosions, called coronal mass ejections, or CMEs. Electrified gas clouds weighing billions of tons raced outward. Solar telescopes recorded the action...

The largest emission of solar x-rays ever seen. The hottest flares, at tens of millions of degrees.

And the fastest reaching speeds clocked at six million miles per hour. The sun became a giant plasma weapon, more potent than any in science fiction and pointed right at our home planet.

On Earth the Halloween storms produced some of the most spectacular auroras ever seen at the north and south poles. They also brought jolts of electricity that caused power outages in Sweden, and disrupted airline navigation.

In space, these storms damaged 28 communications satellites, and destroyed two. And they didn't stop there.

As the energetic surge swept past Mars, it was so strong it burned out the radiation monitor aboard the spacecraft, Mars Global Surveyor. Ironically, this instrument was designed to study radiation that human explorers might encounter on future missions beyond Earth.

Months later, the rush of solar energy washed over the two Voyager spacecraft, on their way to the far reaches of the solar system.

CMEs like these have been known to blast their way out the far edges of the solar system, where the solar wind meets the flow of gas around the galaxy itself. This stormy season on the sun lasted about five weeks. It was by no means the worst.

A solar eruption in 1859 was so powerful it set fire to telegraph offices several people got nasty electric shocks, simply because they were working with metal objects.

And for the next few nights, auroras were reportedly bright enough to read by.

A similar storm today could easily cause more than two trillion dollars in damage to our high-tech infrastructure, twenty times greater than hurricane Katrina.

But believe it or not, the threat is about to get even worse.

We are beginning to change the way we acquire and use energy by expanding our power grid to accommodate wind farms, solar arrays, new nuclear plants and other renewable energy sources.

This grid will get larger and smarter. With microprocessors in most every device communicating and negotiating with one another running everything from air conditioners to power plants.

A sudden surge of solar activity could strike the grid directly inflicting a substantial amount of damage on the emerging smart power economy. To understand what a powerful force the sun can be take a look at Venus. At almost exactly the same size and mass as Earth, it's truly our sister planet.

The thinking is that long ago, fierce solar winds stripped off lighter water molecules from its upper atmosphere. What was left was a witches' brew of acidic gases and carbon dioxide that thickened at lower altitudes, rising to some 90 times the density of Earth's atmosphere.

At a concentration of 95% in Venus' atmosphere, CO2 gas trapped increasing amounts of sunlight and drove surface temperatures close to 1000 degrees Fahrenheit.

How did Earth avoid such a harsh fate?

One of the main reasons is that our planet has what Venus lacks a natural defense against solar attacks.