Nuclear fusion
Stellar evolution
It isn't. Heavier elements are formed in stars as well. Iron is basically the heaviest element for which energy can be gained. When converting iron into heavier elements, energy is lost. However, this doesn't stop supernovae, at a temperature of about one gigakelvin (a billion degrees), to form heavier elements. In fact, the interior of stars is practically the ONLY way such elements can be formed.
Heavier atoms which could not be formed as a result of fusion are produced as the result of a star that has run out of fuel exploding. It essentially forces atoms which do not release energy to fuse together.
Well, Helium is an element, and "substance" can mean a combination of elements and compounds... which are formed according to their chemical properties. Assuming you were asking about pure elements, rather than substances, all elements up to Iron-56 are formed by nuclear fusion, presumably inside of stars. This is due to to Iron-56 being the most stable nucleus in the periodic table. Hydrogen-1 forms Helium-4, Helium combines to form Beryllium-8, Carbon-12, and Oxygen-16. And so on.... up to Iron-56. Fusion of heavier compounds will not result in excess energy (e.g. a star's heat and light) so those nuclear reactions will not sustain a star. Therefore, all elements heavier then Iron-56 are formed by the extraordinary conditions of a super nova. Because earth has abundant elements heavier than Iron-56, it is thought that the matter in our Solar System has been through several solar cycles... In other words, "we are all made of stars".
Nuclear Fusion Processes, those that form new elements, require both high temperatures and pressures. These are only found at or near the center of the star. Hydrogen fusion is the easiest (excepting deuterium but that's a bit too detailed for here) and takes place in the smallest of stars. (Brown Dwarves don't count as no fusion is going on there). For successively heavier fusion reactions creating heavier elements higher temperatures and pressures are required and therefore take place nearer the center of the star within the sphere of hydrogen fusion. The very largest star therefore have an onion like structure where more and more internal layer are forming heavier and heavier elements. The heaviest element that can be formed by energy producing reactions is Iron. After that the formation of even heavier elements absorbs energy. This is why all the elements above Iron are thought to form in Super-novas reactions where there is incredible temperatures and pressures and no worry that energy absorbing reactions are going to shut things down.
They are formed inside of stars.
by the process of nuclear fusion
It is believed this was how the universe formed. Hydrogen fuses to form all the other heavier elements on the periodic table up to Element number 92
Other elements were formed in stars by nucleosynthesis.
Heavier elements are formed from hydrogen, the most abundant element in the universe, through a process called nuclear fusion. There are machines or structures in the universe that do this, and we call them stars. It is the process within stars, stellar nucleosynthesis, that allows heavier elements to be created up through iron. Elements heavier than iron are formed in supernova events. Use the links below to learn more.
hydrogen, helium, and a small amount (less than 2%) of heavier elements
I think it's our Sun which gets heavier elements from fusion of hydrogen and other light elements.Edit: Our Sun does create helium from hydrogen by fusion, but that's all. The reason it has heavier elements is that these come from the nebula that formed the Sun. The heavier elements are thought to have come from stars that exploded as "supernovas", a long time ago.
Chemical elements are formed in the Universe by stellar nucleosynthesis.
First hydrogen nuclei fuse to form helium, and then as the star ages heavier and heavier elements are formed.
Elements that are formed in cool stars are heavy but not heavier than iron. (Elements that are heavier than iron are formed in a supernova.)
Elements heavier than iron are formed in super-nova explosions.
No, hydrogen was. other heavier elements, starting with helium, were formed by the nuclear fusion of hydrogen nuclei.