Translation:The temperature of the Sun is thousands of degrees.
Duolingo's current sentence, "La température du Soleil fait des milliers de degrés" isn't ideal, but we could use "atteint" or "est de" instead of "fait". Read on.
This isn't really the same "fait", or at least "il fait chaud" is a significantly different structure.
To begin with, let's look at how "faire" is used with measurements.
Ce bâtiment fait 30 mètres de haut.
Ce bâtiment fait 30 mètres de hauteur.
This building is 30 metres tall.
Ce lit fait 2 mètres de long.
Ce lit fait 2 mètres de longueur.
This bed is 2 metres long.
Ma chambre fait 4 mètres de large.
Ma chambre fait 4 mètres de largeur.
My bedroom is 4 metres wide.
Cette piscine fait 5 mètres de profondeur.
This swimming-pool is 5 metres deep.
This is also what we're dealing with when talking about the measured temperature of a thing. By "thing" I mean a specific real subject, and as such, it's not the same grammatical concept as used in "il fait chaud" when talking about the weather, where "il" is an impersonal "dummy" subject, and doesn't refer to anything in particular.
However, note that to match the structure of Duo's current sentence, the structure of my examples would change to:
La longueur de ce lit fait 2 mètres.
And unfortunately, it's actually weird to apply "fait" in this way, where the subject of the verb is the aspect being measured (longueur, température), and not the object itself (ce lit, le soleil).
However, with the sentence turned around like this, the verb can be changed to "être" with the addition of "de":
La longueur de ce lit est de 2 mètres.
And we can indeed say:
La température du soleil est de milliers de dégrées" ["est de + des" = "est de"].
This would be more likely, and it's a better sentence, as confirmed by Sitesurf in other discussions.
And we can also say:
Le soleil fait des milliers de degrés (de température).
Le soleil a/fait une température de milliers de degrés.
Finally, we can also change Duo's verb to "atteint":
La température du Soleil atteint des milliers de degrés.
In other words, not only is Duolingo's current sentence, "La température du Soleil fait des milliers de degrés", not analogous to "il fait chaud", but it would actually be a good idea not to use the sentence at all, because it's quite unusual, if not completely wrong. However, we could save it by substituting "atteint" or "est de" for "fait" (per Sitesurf).
Probably for the same reason we capitalize Sun and Earth in English, they are the proper names for our sun and our planet. Otherwise any other star would be a sun and anything involving terrestrial components like the earth under my feet, earthenware etc. would be lower case.