- View images of the Sun<\/a>, or find out how to safely observe the Sun<\/a>.<\/strong><\/em><\/li>\n<\/ul>
![\""A\"]( "\\"\"A\\"\/")
<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>A to-scale representation of the Astronomical Units of distance from the Sun to Saturn. The distance to Earth\u2019s orbit is 1AU. Credit: Mark Garlick \/ iStock<\/span><\/figcaption><\/div>\n Earth lies about 150 million km from the Sun, and this distance \u2013 known as an Astronomical Unit \u2013 is used to measure large-scale distances between other bodies.<\/p>\n
Neptune<\/a>, for example, is about 30 AU from Earth.<\/p>\n
The Sun has a radius of about 696,000km and is big enough to hold over 1 million Earths!<\/p>\n
Without the Sun, we would be nothing: it is the source of life on our home planet.<\/p>\n
And yet, for those of us who aren\u2019t astronomers or solar scientists, it can sometimes feel like we know more about Mars<\/a> or the gas giants Jupiter<\/a> and Saturn<\/a> than our own host star.<\/p>\n
![\""An\"]( "\\"\"An\\"\/")
<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>An active region on the Sun, as seen by NASA\u2019s Solar Dynamics Observatory. This image shows a sunspot in visible and extreme ultraviolet light, with particles seen spiralling along magnetic field lines. Credit: NASA\u2019s Goddard Space Flight Center\/SDO<\/span><\/figcaption><\/div>\n There is much yet to be discovered about the Sun, but solar astronomers have been able to answer some fundamental questions about its structure, what makes it tick and its influence on the rest of the Solar System.<\/p>\n
Here are some of the biggest questions about the Sun and solar science, and their answers.
1<\/span>What sort of star is the Sun?<\/h3>\n<\/div>
\u00a0<\/p>\n
![\""The\"]( "\\"\"The\\"\/")
<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>The Hertzsprung-Russell diagram gives us insights into how stars evolve<\/span><\/figcaption><\/div>\n The Sun is classified as a G-type main sequence star. The \u2018main sequence\u2019 part of the name refers to the fact that the Sun is at a stage in its life when the primary fuel source is the conversion of hydrogen to helium.<\/p>\n
Stars in this phase lie on what we call the main sequence of the Hertzsprung-Russell (HR) diagram (seen above).<\/p>\n
This diagram is a graph of stars\u2019 absolute magnitudes versus their surface temperatures.<\/p>\n
Stars spend the majority of their lives on the main sequence; the exact duration depends on the mass of the star.<\/p>\n
And the main sequence is the most prominent curve on this diagram, forming a slanted \u2018S\u2019.<\/p>\n
The main sequence is not a sharply defined line on the HR diagram, as several other properties like chemical composition and age influence the location of a star on the diagram.<\/p>\n
![\""Artist's\"]( "\\"\"Highly\\"")
<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>Artist\u2019s illustration of a neutron star. Credit: Pitris \/ iStock \/ Getty Images Plus<\/span><\/figcaption><\/div>\n A main sequence star is in a state of hydrostatic equilibrium \u2013 the internal pressure created by the nuclear reactions at the star\u2019s core balances the inward pull of gravity, and the star is stable.<\/p>\n
But once hydrogen burning in the core ceases, the star loses this stability and evolves away from, or off, the main sequence.<\/p>\n
The star swells up and becomes a red giant or supergiant. The surface temperatures of these stars are relatively cool since they are expanding, but they are very luminous, so they are plotted on the HR diagram above and to the right of the main sequence.<\/p>\n
Eventually, the star will reach the end point of its life and become a white dwarf, neutron star or black hole \u2013 again depending on its mass.<\/p>\n
Only white dwarfs are normally plotted on an HR diagram.<\/p>\n
2<\/span>What is inside the Sun?<\/h3>\n<\/div>
\u00a0<\/p>\n
![\""A\"]( "\\"\"sun's+interior+credit+Newton+Science+Magazine_518fb096-986b-4882-8493-7f51c46d03ba-prv\"\\"\/")
<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>A cross section of the Sun\u2019s inner portions. Image Credit: Newton Science Magazine<\/span><\/figcaption><\/div>\n At the centre of the Sun is the core, which produces energy by nuclear fusion, converting hydrogen into helium.<\/p>\n
Energy travels into the radiative zone above the core, which typically keeps it trapped for thousands of years.<\/p>\n
Above the radiative zone is the convective layer. Making up around 30% of the interior, this transfers energy to the surface (the \u2018photosphere\u2019) by the process of convection: gas rises and falls just like blobs of wax in a lava lamp.<\/p>\n
The photosphere has been studied using ground-based solar telescopes.<\/p>\n
Surrounding the Sun is its atmosphere \u2013 the solar corona.<\/p>\n
Accurate measurements and studies of the corona have been made by ground-based studies, during solar eclipses<\/a> and by space-based observatories.<\/p>\n
Data from space missions also enables astronomers to measure the vast solar magnetic field, which helps to refine models of our star\u2019s structure.<\/p>\n
3<\/span>How do we know what\u2019s inside the Sun?<\/h3>\n<\/div>
\u00a0<\/p>\n
![\""Helioseismologists\"]( "\\"\"helioseismology\\"\/")
<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>Helioseismologists measure the speed at which acoustic waves travel through the Sun\u2019s interior.<\/span><\/figcaption><\/div>\n In order to understand the structure of the Sun, astronomers use a mixture of observation, theories and computer modelling to come up with a picture that best describes the characteristics that have been recorded.<\/p>\n
Much of our knowledge of the solar interior derives from helioseismology.<\/p>\n
Like terrestrial seismologists, helioseismologists use oscillations to probe the interior of a body: seismic waves generated by earthquakes in the case of the Earth; pressure waves within the convective zone in the case of the Sun.<\/p>\n
These waves cause vertical motions at the Sun\u2019s surface, the photosphere. Observing the oscillations reveals the properties of the different layers.<\/p>\n
The waves within the Sun can be best described as a continuum and many sources produce a constant acoustic ring.<\/p>\n
At the solar surface, these waves produce tiny up and down motions in the gases, which can be seen as Doppler shifts in high resolution spectra.<\/p>\n
The temperature, composition and other properties of the solar interior strongly influence the behaviour of these waves.<\/p>\n
Combining such observations with theories about star structure allows astronomers to produce models of the Sun\u2019s internal composition.<\/p>\n
We see three types of waves in the motion of the Sun\u2019s surface: acoustic, gravity and surface gravity waves.<\/p>\n
The oscillations have wavelengths ranging from thousands to hundreds of thousands of kilometres.<\/p>\n
4<\/span>Was the Sun born in a star cluster? Where are the other stars?<\/h3>\n<\/div>
\u00a0<\/p>\n
![\""One\"]( "\\"\"One\\"\/")
<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>One of the most famous star clusters in the night sky: the Pleiades. Credit: Pete Lawrence<\/span><\/figcaption><\/div>\n Like most stars that we see in the night sky, the Sun isn\u2019t gravitationally bound to a cluster of stars.<\/p>\n
However, astronomers believe that the Sun (like most stars) was born as part of a cluster.<\/p>\n
The most massive stars in that cluster would now have exhausted their fuel and died.<\/p>\n
Over its 4.6-billion-year lifetime, the Sun has completed around 20 orbits of our Milky Way<\/a> Galaxy, and the gravitational forces acting on the original cluster members have long since dispersed the Sun\u2019s siblings.<\/p>\n
At present, the Sun appears to be located within the Ursa Major Moving Group \u2013 a loose group of stars that share the same age, metallicity, radial velocity and proper motion.<\/p>\n
However, the Sun isn\u2019t actually a member of this young (approximately 300 million years old) grouping.<\/p>\n
5<\/span>Where is the Sun located in the Milky Way?<\/h3>\n<\/div>
\u00a0<\/p>\n
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<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>The Sun is currently thought to be about 65 lightyears (or 20 parsecs) above the galactic plane<\/span><\/figcaption><\/div>\n Observations of the distribution of neutral hydrogen gas in the 1960s indicated that our Sun sits somewhere above the mid-plane of our galaxy, the Milky Way.<\/p>\n
Since then, a variety of techniques have been used to try and estimate just how far above the galactic plane the Sun actually sits.<\/p>\n
Most measurements are based on statistical studies of the distribution of stars, clusters, gas or dust around the Sun, and estimates have ranged from only 6 parsecs (1 parsec is about 3.26 lightyears) to as much as 42 parsecs.<\/p>\n
The most recent estimates have suggested a likely distance of around 20 parsecs, or 65 lightyears.<\/p>\n
This distance is not constant, however, as the Sun rises and falls as it completes its 230-250 million year orbit around the Galaxy.<\/p>\n
It is estimated that the Sun undergoes 2.7 oscillations per \u2018galactic year\u2019, with the last passage through the plane being about 3-5 million years ago.<\/p>\n
The maximum distance the Sun can get above or below the plane is thought to be 50-100 parsecs (between 163 and 326 lightyears).<\/p>\n
6<\/span>What heats the Sun\u2019s corona?<\/h3>\n<\/div>
\u00a0<\/p>\n
![\""Miniature\"]( "\\"\"Miniature\\"\/")
<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>Miniature solar flares in the Sun\u2019s corona, by Solar Orbiter, 14 DECEMBER 2021. Credit: Solar Orbiter\/EUI Team\/ESA & NASA<\/span><\/figcaption><\/div>\n The corona is part of the Sun\u2019s outer atmosphere and is composed of ionised gas.<\/p>\n
The Sun\u2019s visible surface (photosphere) has a temperature of about 5,800 Kelvin, but the corona has maximum temperatures between 1 and 3 million Kelvin.<\/p>\n
However, the corona is extremely tenuous, with a density only a thousand-billionth that of the photosphere.<\/p>\n
It also gives off very little visible light, and can only be seen with a coronagraph or during a total eclipse<\/a>, when the bright photosphere is hidden from view.<\/p>\n
But what makes the corona so hot? There are currently two main theories for the \u2018coronal heating problem\u2019.<\/p>\n
![\""The\"]( "\\"\"Solar\\"")
<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>The Solar corona during the 21 August 2017 total solar eclipse by Alex Conu, Castle Garden, WY, USA. Equipment: Canon EOS 5D Mark III, Pentax 75 SDHF refractor<\/span><\/figcaption><\/div>\n One theory proposes that magneto-acoustic waves (sound waves modified by a magnetic field) and Alfv\u00e9n waves (ions oscillating in a magnetic field within a plasma) transfer energy from inside the Sun.<\/p>\n
Another theory argues that it is rather the Sun\u2019s magnetic field<\/a>, which induces electrical fields in the corona, that causes the heating.<\/p>\n
These fields then collapse suddenly, releasing energy into the corona and heating it.<\/p>\n
Theorists and observers are still trying to determine which mechanism is responsible for the coronal heating problem, but it does seem clear that magnetic fields play a key role.<\/p>\n
7<\/span>What is the Sun butterfly diagram?<\/h3>\n<\/div>
\u00a0<\/p>\n
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