The star (or stars - it appears to be a multi-star system) is among the brightest in the galaxy, with the primary component more than 5 million times brighter than the sun. At 7,500 light years away, it’s one of the most distant stars we can see without a telescope, and was the second brightest star in the night sky for a few days in March 1843.
Eta Carinae’s main component is highly active, and spewed a huge eruption of material in the mid-1800s. It was this eruption that made it so bright. It then faded so much that for a while a telescope was needed to see it. Now it’s visible again with the naked eye.
Scientists think it’s likely to explode in a supernova sometime in the near future (astronomically). That means anytime between now and say, 3 million years from now. If it does, it will be one of the brightest objects in our sky for a short period. Probably five times brighter than the planet Venus.
Despite its great distance, it might be one of the most dangerous objects in the sky, because it’s likely to be massive enough to explode as a “hypernova,” Some scientists predict it could strip the Earth of its ozone layer, something that wouldn’t normally happen unless a supernova were within 50 light years of Earth.
The Eta Carinae system is seen below. The material expelled from it during its eruption in the mid-1800s is quite evident. It’s this material that caused the star to fade in brightness from our vantage point for a while, as the material hid the star.
If the sun suddenly became as luminous as Eta Carinea (this can’t happen), the Earth’s entire surface would melt, mountains and all.