A sister's regret

After Luna's return, both she and Celestia shared almost every single possible moment with each-other. After all, it had been a thousand years since this was possible. All seemed to be well again but since her trip to Ponyville on Nightmare Night, Luna was a bit reluctant to doing so. Celestia wasn't completely sure why but it almost felt like Luna was starting to avoid her, only being with her during dinner and other royal duties which required both alicorns to be present. This went on for quite a while until one night when Celestia had stayed awake a longer than usual because of a late letter from her student Twilight Sparkle. She had just finished reading it and was now making her way towards her room, her thoughts still focused on Twilight's latest lesson about how you are always able to share your secrets with your friends, no matter the issue. She made her way through Canterlot castle towards her room. Through the greatly decorated marble hallways and the greatly adorned galleries.  The interior of Canterlot castle really was as beautiful as it looked on the outside. As she walked through a particularly big hallway, she passed her sister's room and noticed that the door was slightly ajar. Through it, she managed to catch a glimpse of her sister who sitting on her balcony. Since it was the first time she saw her sister again since dinner, thoughts about her  being avoided filled her mind again up to the point where she just had to know whether her suspicions were fair or not. Against her better judgement, she entered the room, knowing Luna wouldn't notice her straight away as she was too busy looking up at the sky.
Taking all of her courage, she kindly said "What'cha looking at?"
Luna, a bit startled by the sudden voice, looked behind her and upon noticing her sister, she said "Celestia, I didn't know you were still awake."
It was pleasing to hear that Luna was starting to speak as everypony else now after a lot of practicing but still, Celestia couldn't help but feel as if it sounded uneasy and evasive. She chose to ignore it for now. "Twilight's letter came in a bit later today."
"Ah" Luna said and she went back to staring at the sky.
"So?"
"So?"
"So, what'cha looking at?"
"The stars"
"But you're only looking in one direction"
"One star then"
Not once did Luna look back down to face her sister. It hurt Celestia a bit to see her sister act like this but she decided to once again ignore it.
"Well, what's so special about that one star then?" Celestia asked, her voice filled with kindness.
"Not much" Luna replied coldly.
It was apparent that Luna didn't want Celestia in her room right now, confirming Celestia's suspicions. Still, she wasn't going to back down easily.
"Then why are you staring at it?" Celestia tried.
"Just because"
"Come on, there must be one reason why you're looking at that star" Celestia said, all the while staying kind towards her sister.
Luna sighed and pointed her hoof at the telescope on her balcony which was facing in the same direction Luna was staring. "See for yourself then"
Feeling a bit uneasy, Celestia left the room and stepped on the balcony to join her sister. The balcony was made from pure white marble just like the rest of the castle, yet it was decorated by all kinds of dark-blue ornaments, indicating that this was Luna's balcony. She made her way towards the telescope and  looked through it. What she saw amazed her. "Wow, what a beautiful red star" she said excitedly. She stared at it a little while longer until Luna spoke again.
"It's dying"
Celestia looked up from the telescope with disbelief. "What?"
"it's in the last phase of its lifespan"
"What do you mean?" Celestia asked curiously.
"When a star gets old, it swells up to immense proportions and turns red. Then, after a while, it gets too big and it explodes." Luna said, a little warmer this time.
"That's a sad thought" Celestia said.
"It happens" Luna said, going back to her cold voice again.
Celestia, noticing this, tried to keep up the conversation "You say that this star will grow really big... it's still just a small dot in the sky."
Luna sighed again "Celestia, you know that the sun is just as much of a star as all of the others."
"Well yes, I know that but I don't see how..."
"The sun just happens to be really close by whereas the other stars are incredibly far away."
"Really?"
"Why yes, in fact, that red star probably already died a long time ago."
"It did?" Celestia asked curiously, noticing Luna's mood lift up a bit as she was talking about the night's sky. Still, it was kind of held back it seemed.
"The light coming from the stars has to travel quite some distance before it reaches Equestria. The star as we see it right now is actually the star as it was millions of years ago."
"Wow, that's amazing" Celestia giggled. "You always were the smartest one." When Luna didn't reply, Celestia continued talking. "So, the light from the sun takes a while to get here too then?"
"Yes, it does, only much shorter. I'd say it takes just a few minutes"
"A few minutes? How do you know that?"
"You can see quite a lot from the moon" Luna answered, her voice suddenly very cold again.
Of course, Celestia knew perfectly well why her sister had been avoiding her. She had tried very hard to not mention it at all anymore but a thousand years aren't ignored easily. She looked at Luna who had gone back to staring at the red star in the sky. Grief filled her already saddened heart as she tried to find the correct words. She couldn't act nice and happy anymore.
"Luna, I..." Celestia started, her voice shaking.
"Oh no, don't start with me like that, Celestia. I deserved what I got. I let my ego and my pride get the better of me and turned into Nightmare Moon" Luna cut her off.
Celestia's eyes went wide with disbelief.
"When I said I was sorry for what I did, I truly meant it. I was sorry for what I did and for what I became."
Her sister taking all of this blame upon herself was all Celestia could take. She had never intended on having Luna apologize when she transformed back. She just wanted to find out whether Luna was her old self again or not. She just wanted to make sure that she'd gotten her sister back. When Luna apologized, Celestia was slightly taken aback, but she didn't show any of it. She was foolish in doing so and she knew it, yet she stayed silent on the matter. Tears started filling her eyes.
"Luna, I... You didn't have to..."
"Oh but I did. I tried to make the night last forever which, if I did succeed, would have been disastrous."
It tore Celestia apart to hear her sister talk to her like this. A tear flowed down her cheek  as a voice in her head told her to leave Luna alone. Her sister who had been staring at the star non-stop clearly wanted to be left alone. She was going to follow this voice had it not been for her stubborn will to stay.
"You knew what could happen and you were right in punishing me by locking me up in t-"
"Stop it!" Celestia screamed, her voice shaking.
Luna was taken aback by this and looked at her sister, only now noticing her tear-filled eyes.
"Stop saying you're sorry! If there's anypony who should be sorry it's me!" Celestia screamed, her eyes closed as tears streamed down her face. "I was a fool! I should never have done what I did! I just couldn't think of a way to stop you and I turned to the only method I knew of: the elements of harmony!"
Luna silently stared at her sister with big eyes while Celestia was looking steadily at her hooves with a few glances upwards every now and then.
"Right after I sent you to the moon, I wanted to undo it! I wanted you back again right away but I just couldn't! Whatever I tried, I couldn't get you back!"
By now, Celestia was too afraid to look her sister in the eyes. She was afraid of what she would see. She hated herself for this yet she couldn't bring herself to look up.
Luna on the other hand looked on in shock as her sister poured her heart out at her. She had no idea that her behaviour as of late would result in this. Sure, she admitted that she started holding a small grudge against her sister for locking her up in the moon. Her trip to Ponyville made her realize how much she'd missed during her imprisonment and how scared the ponies had been of her at first but it came with the realization that it was her own fault. If she hadn't turned into Nightmare Moon, all of this wouldn't have happened.
"I wish there was a way for me to repay you every second of those thousand years," Celestia continued, "but I know fully well that there just isn't! I should never have done what I did! You said you were sorry, but your apology was nothing compared to the regret that I've been feeling all this time!"
At this, Celestia halted her words. She closed her eyes and held her face downwards. She was ready to collapse then and there. Nothing mattered to her anymore. Not even thinking about her student Twilight Sparkle could make her smile again. She thought about what would happen next, what she were to do now when she heard Luna's hooves make their way past her towards her room. Of all the ways Luna could have reacted, this one was the most unbearable: the silent treatment. Still, Celestia deserved it, or so she felt. She had brought this upon herself. She even started wishing that Luna would close the door and lock her out but to her surprise, the sound of a door closing didn't come. In fact, the sound of Luna's hooves seemed to be coming closer again, only it seemed to be coming from her other side this time. They came closer until they suddenly stopped right next to her. Hesitantly, Celestia opened her eyes and indeed saw her sisters hooves standing to the left of her. It confused Celestia but she didn't dare look up.
That was when Luna lowered her body and lay down on the floor. She closed her eyes and just lay there next to her sister.
Celestia's eyes widened as more tears filled her eyes again. She had expected her sister to do anything but this. At first she wanted her sister to yell at her, to scream out all of her frustrations but after seeing Luna laying next to her, she felt compelled to lay down there with her, just like they had done so many times before. Slowly, she lowered herself and lay down. She too closed her eyes as a smile crept over her face.
"Goodnight, Tia" Luna whispered warmly. It had been a thousand years since the last time her sister had called her Tia and she was so happy to hear it once again.
"Goodnight, Lu" Celestia whispered back as both sisters slowly fell asleep.