How was the Sun’s corona first discovered?

The Corona, the outermost region of the Sun’s atmosphere, is rendered visible to observers during a total solar eclipse. This rarefied solar layer can only be observed when the solar disc is entirely blocked. Even though the Corona extends millions of miles into space and its estimated temperature exceeds one million Kelvin, it is also quite diffuse: about 10 million times less dense than the material contained within the photosphere, often mistakenly referred to as the Sun’s “surface.”

So, this corona has been seen throughout human history. However, the first person who was said to have recognized this material as having been part of the Sun rather than the moon was the Italian-French astronomer Giacomo F. Maraldi (1665–1729).

Spanish astronomer Jose Joaquin de Ferrer y Cafranga (1763–1818), the man who coined the term “Corona,” observed a total solar eclipse in 1806 and likewise believed that this light was part of the Sun’s atmosphere and was not associated with the moon. “Corona” is taken from the Latin word for “Crown.” French astronomer Pierre Janssen (1824–1907) provided the first observational evidence supporting the notion that the Corona was part of the Sun by noting alterations in the coronal intensity in relation to changes in the sunspot cycle.

Tangential note: It was by studying the corona that Sir Joseph Norman Lockyer (1836–1920) discovered the second element, which had not yet been observed on Earth. He named his element “helium,” after “Helios,” the Greek god of the Sun.

The solar corona is only visible during a total solar eclipse. As seen in the photograph above, the Corona appears as a diffuse “ring” of light surrounding the eclipsed Sun. Image: Luc Viatour taken of the 11 August 1999 eclipse.

 

Would it be possible to 'move' the Earth sometime in the future, while keeping everything on Earth going as it is, before the Sun gets too big?

Greetings!

First, I should mention that we would need to “move” Earth prior to the stage at which the Sun expands to the red giant stage. This transformation is due to occur in 5–6 billion years, when the Sun exhausts its core hydrogen reserves. However, the Sun’s luminosity (energy output per second) is slowly increasing as a consequence of the thermonuclear core reactions. Astronomers estimate that the Sun’s luminosity increases by 6% every billion years. Consequently, Earth will be rendered uninhabitable in approximately 1.1 billion years.

In order to keep Earth habitable, we’d have to move to the Sun within 1.1 billion years and continuously shift its position away from the Sun because the habitable zone, the region in which temperatures are conducive to life’s survival, would also expand away from it. By the time the Sun becomes a red giant, the habitable zone will have extended all the way out to the orbits of Jupiter and Saturn. (Jupiter’s average heliocentric distance is 483 million miles; Saturn’s mean distance is 914 million miles.) See graphic below.

The changing “Habitable Zone.” Earth is presently located within Earth’s habitable zone, hence our continued existence on it. However, as the Sun’s luminosity increases, this zone will expand away from the Sun. Earth will be rendered uninhabitable in about 1.1 billion years. By the time the Sun expands to the red giant stage, the habitable zone will be hundreds of millions of miles farther away: in the region where Jupiter and Saturn revolve around the Sun. Image: Astronomy Magazine

A few physicists have actually considered the possibility of shifting Earth to protect it from the evolving Sun. One can well imagine that shifting Earth safely constitutes one of the most challenging astro-engineering problems. Perhaps the most feasible option discussed so far would be to direct comets and asteroids with widths exceeding 100 kilometers around Earth and then to either Jupiter or Saturn. The “pull” caused by these repeated revolutions, though quite small, could cumulatively direct Earth gradually but inexorably away from the Sun. Of course, this solution poses many problems, namely, maintaining control of these bodies to ensure that none of them eventually crashes onto Earth. Considering that the impact of a 10-km wide asteroid ended the Cretaceous period, the consequences of a 100-km wide asteroid -taking into account the 1:10 ratio between the impacting body’s diameter and that of the produced crater- could put an end to Earth life itself.

Fortunately, we have plenty of time to contemplate the matter. That last statement presumes that our species will even persist for 1.1 billion years -4,400 times longer than the present duration of homo sapiens. If humans are still extant by this time, presumably they would have devised a solution to either shift Earth away from the Sun or to actually move to another star system altogether. While the latter option would be the logistically easier of the two, who knows what technology will develop in the intervening time. After all, humans are known for their problem-solving and, admittedly, problem-creating, capacities.

 Since the Milky Way is spinning, if we tried to cross it in a ship, would we just end up back where we started? In other words, would the rotation of the Galaxy cause us to run right back into earth along the way

Intriguing question. The answer of where you would end up depends entirely on the speed and direction of your ship. To maintain our sanity, we will neglect the harrowing effects of venturing close to Sagittarius A*, the supermassive black hole in the galactic nucleus. Astronavigation, particularly on an interstellar scale, is quite tricky because all the objects within the Milky Way are rapidly moving. Conversely, if, let’s say, you decided to manufacture a boat out of leather so you could sail across the North Atlantic from Ireland to Newfoundland. You could expect a chilly odyssey, but at least Newfoundland would remain in place. (We’re neglecting the geological shift because it is conveniently negligible.)

The galaxy is in constant motion. However, unlike a merry-go-round or a vinyl record, the galaxy doesn’t move as a single rigid body. The stars are all moving either independently or in clusters. Regard the following graphic which shows the location and direction of star streams within our region of the galaxy.

So, one would have to define the “same place.” In this instance, one could refer to the local standard of rest: an “average” of all the motions of stars within one’s vicinity. Of course, this local standard is difficult enough to establish within one local region of the galaxy, let alone the entire Milky Way. One could also refer to the “galactic plane,” which would be the average of the Milky Way’s “thickness” and its diameter. Measuring the Milky Way’s thickness, which has been estimated at 1000 - 3000 light years, is particularly challenging because the boundaries are vague and always changeable as the stars are all moving along undulating paths within it, like horses on a Merry Go Round.

As the ship’s captain, you decided to traverse the Milky Way so as to reach the other side. Well, if you define the other side as the star whose galactic coordinates are 180 degrees from Earth’s, you’d experience difficulties because that star would move well away from this position by the time you reached it. Even if your craft could approach light speed, this star would be out of place as your journey’s distance would equal about 75,000 light years. (Remember, we’re not at the galaxy’s outer edge, but are instead about 25,000 light years from the nucleus.)

Let’s say, however, that you can know precisely the point directly opposite our solar system relative to the nucleus along the galactic plane. You would not encounter Earth unless you happened to be moving so slowly that the journey required about 112 million years. Were this to be true, you MIGHT encounter our solar system, which requires about 225 million years to complete one revolution around the galaxy. But that “might” in bold 100-point print because the chances are vanishingly low. Remember that according to the most recent estimates, our galaxy contains about 400 billion stars.

Your aim would have to be superb to hit Earth. However, the galaxy, itself, wouldn’t spin you back here. You’d have much more control over your trajectory than that.

How can something so big as the sun be held together by gravity?
The same way that something as unfathomably large as a galaxy, galaxy cluster or supercluster can be held together by gravity: the matter contained within. Yes, with an 865,000 mile diameter, the Sun’s volume is enormous. So, too, is its mass. The Sun contains about 99.86% of all the material within the solar system. (It is 333,000 times more massive than Earth.) We often say that the massive particles are attracted to one another gravitationally. One could also describe the situation from an Einstein perspective by stating that all this matter deforms its local space-time geometry to create a “gravity well” that holds the Sun together.

If, for instance, one were to expand Earth to the size of the Sun while the mass remained constant, the material would dissipate because the gravitational bonds connecting all the constituent particles would become too tenuous. Gravity is the weakest of the fundamental forces (gravity, electromagnetism, and the weak and strong nuclear force.) If you doubt this assertion, look at your refrigerator magnet. The electromagnetic field generated by that tiny magnet is easily overwhelming the gravitational pull of an entire planet. However, as comparatively weak as it is, gravity becomes powerful once you concentrate enough material within a small enough region.